The thymoleptic drug citalopram (CIT) belongs to the selective serotonin reuptake inhibitors (SSRIs) and is today extensively used in psychiatry. Further clarification of the enantiomer-selective distribution of racemic CIT in both clinical and toxic doses is highly warranted. By a steady-state in vivo paradigm, rats underwent chronic systemic exposure for 10 days by using osmotic pumps and the total as well as the individual distributions of the S- and R-enantiomers of CIT, and its metabolites in serum and two different brain regions, were analysed. In serum, the S/R ratios in the groups treated with 10, 20, or 100 mg kg−1 day−1 were 0.94, 0.83, and 0.34, respectively. The ratios were almost the same in the brain regions. In the group treated with 100 mg kg−1 day−1, the serum and brain total CIT levels were found to be 20 times and 6 – 8 times higher than in the rats treated with 10 or 20 mg kg−1 day−1, respectively. In all groups, the CIT levels were higher in brain tissue as compared to serum. In a spontaneous open-field behavioural test, a correlation between clinical and toxic drug concentrations was observed. In conclusion, the R-enantiomer was present in an increased proportion compared with the S-enantiomer when higher steady-state CIT concentration was prevailing. This is of particular interest, since the S-enantiomer is responsible for the inhibition of serotonin reuptake in vitro. The present data may be of importance, as full understanding on where different racemic or enantiomeric drug effects of CIT and its main metabolites are unravelled. Keywords: Citalopram, enantioselective drug analysis, open-field behaviour, pharmacokinetics, toxic dose Introduction Modern pharmacotherapy is one of the cornerstones in the treatment of affective psychiatric disorders, including major depressive disorder (MDD), the latter being a well defined entity belonging to one of the overall largest morbidity/mortality health care found in modern medicine (Boerner & Moller, 1999). During the last decade, the clinical use of monoamine-active drugs has increased. Today there is fortunately access to a great variety of this type of drugs to counteract MDD as well as other affective diseases, e.g. obsessive compulsive disorder, panic attacks, and other anxiety disorders (Preskorn, 1995; Rickels & Schweizer, 1990). Pharmacodynamically (PD), these thymoleptic drugs act directly through affecting/enforcing the monoamine neurotransmitter systems, often primarily by a potent reuptake inhibition of neuronally released serotonin (5-hydroxytryptamine; 5-HT) and/or noradrenaline (NA). Of modern thymoleptics, the selective serotonin reuptake inhibitors (SSRIs) are a group of drugs often considered as first-line therapy for treating MDD due to a clinical combination of a rather advantageous side-effect profile and documented efficacy compared with older monoamine-active drugs like the tricyclic/tetracyclic antidepressants (TCAs) (Andrews & Nemeroff, 1994). Despite a general positive clinical psychopharmacological development during the last decade, several relevant pharmacokinetic (PK) factors for the more recent thymoleptics have been grossly neglected for an adequate scrutiny in living paradigms. Such factors include the definition of the concentrations of both racemic and enantioselective drug components in different body compartments, e.g. in blood and brain, both of parent drug and its major metabolites. It has been pointed out that focusing only on PK outcome of the total sum of two enantiomers and such concentration-effect relationship, may lead to the generation of erroneous data and misleading interpretations for the individual enantiomers occurring in vivo (Evans et al., 1988). In this context, it is noteworthy that for almost a decade now the most commonly used thymoleptic drug in Sweden, citalopram (CIT; Cipramil®) (Personne et al., 1997), is administered as a racemate without sufficient data available on the enantiomer-specific outcome of this procedure. Further, CIT is considered the most selective of the currently clinical available SSRIs, having no, or very little, affinity for NA or dopamine uptake sites, and CIT is believed not to inhibit monoamine oxidase (Hyttel et al., 1995). Still, the two enantiomers of CIT are known to possess somewhat different degrees of PD-activities (Hyttel et al., 1992), and the major CIT metabolites, demethylcitalopram (DCIT) and didemethylcitalopram (DDCIT), are also known to encounter 5-HT reuptake inhibiting properties (Hyttel, 1977). From the scarce literature available, it seems that mainly the S-enantiomers of CIT and DCIT are responsible for the clinically relevant 5-HT reuptake inhibiting properties of the drug (Hyttel et al., 1992). Further scrutiny of the enantiomeric outcome in steady-state conditions is therefore highly warranted. Even if the SSRIs are considered less toxic compared with the TCAs, risks of developing serious side effects such as ECG abnormalities and convulsions have been evidenced for CIT as a consequence of larger ingested doses by patients (Grundemar et al., 1997; Personne et al., 1997). Furthermore, fatal overdoses have been reported where CIT alone was the cause of death (Worm et al., 1998; Ostrom et al., 1996). Overall, very little data are available describing toxicity aspects for any of the presently extensively used SSRIs when administered in high/toxic doses to rats, although recently toxicokinetic data for older types of antidepressants (i.e. TCAs) are being published (Darcy et al., 1999). The main aim of the present investigation was to acquire enantiomer-specific data from blood and brain in clinically relevant and toxic doses of the commonly used SSRI CIT after chronic systemic exposure of the racemic compound by using osmotic pumps. Further, an overall estimate of the drug distribution between serum and brain in steady-state were sought on these doses. To do this, an in vivo rat experimental setting was used for practical, ethical, and economical reasons. By this study design, the interpretation of data had to focus on a conceptual rather than strict conclusive level in relation to the relevant clinical context. Finally, using the in vivo animal paradigm, the main clinical outcome of the sustained drug challenges was documented by recordings of spontaneous open-field behavioural performances.