Treatment with antidepressants is regularly demanded in post partum women due to the high prevalence of mood and anxiety disorders during this period. In general treatment with selective serotonin reuptake inhibitors (SSRIs) have been considered to be compatible with breast-feeding [1, 2] although some concern has been alleged about the use of fluoxetine because of three reports of possible adverse effects in suckling infants whose mothers were treated with fluoxetine [3–5]. The subject of this report is a 31-year old woman with a history of panic disorder but no somatic disease as assessed by medical history, physical examination and routine blood chemistry. Height and body weight were 1.75 m and 90 kg, respectively. The study was performed 3 months post partum. She had then been treated with fluvoxamine for 6 months and for 1 week with the present dosage, 100 mg twice daily. After intake of 100 mg fluvoxamine (Fevarin, enteric-coated fluvoxamine maleate, Solvay Pharma, Brussels, Belgium) at 08.00 h venous blood samples (10 ml) and milk samples (5 ml) from both breasts were collected every hour during a 12 h period. Fore milk samples were obtained using a manual breast pump. The first 2 ml of the breast milk was discarded. All milk and serum samples were stored at −20° C until analysis with a previously described high performance liquid chromatography technique [6]. The limit of quantification was 100 nmol l−1 and 50 nmol l−1 for serum and breast milk, respectively, and the method was linear up to at least 3000 nmol l−1 and 1500 nmol l−1 for analysis in serum and breast milk, respectively. The intra-assay coefficients of variations for fluvoxamine were 3.1% in serum at 500 nmol l−1and 2.1% in milk at 500 nmol l−1. The following equation was used in order to calculate the absolute daily dose of fluvoxamine ingested by the newborn per kg body weight: Dinf = ′Css(milk) × Vmilk where ′Css(milk) is the average steady state concentration in milk and Vmilk is the daily volume of milk ingested by the newborn, which was assumed to be 0.15 l kg−1 body weight. ′Css(milk) was calculated as the area under the concentration curve (AUC)/12 h. AUC was calculated by using a noncompartment model in the pharmacokinetic program package WinNonlin, version 1.1 (Scientific Consulting Inc., North Carolina, USA). The relative daily fluvoxamine dose ingested by the newborn (Drel) was calculated using the equation: Drel = Dinf/Dmat × 100 where Dmat is the maternal daily fluvoxamine dose kg−1 body weight. Concentrations of fluvoxamine in serum and milk are presented in Figure 1. The time course of fluvoxamine in milk roughly paralleled the serum time profile. Mean of right and left breast milk to serum concentration at each time point ranged between 1.06 and 1.59. The overall milk to serum concentration ratio based on AUC values was 1.32. The absolute daily dose of fluvoxamine ingested by the newborn was calculated to be 48 µg kg−1 and the relative dose was calculated to be 1.58% of the weight—adjusted maternal dose. Figure 1 Concentration-time profile in milk (▪) and serum (▴) during a dose interval of 12 h at steady state in a subject treated with fluvoxamine 100 mg twice daily. The milk concentrations are average values from the milk samples from both breasts. ... Excretion of fluvoxamine in human breast milk has only been documented in two cases before and was based on single pairs of samples only. In these cases milk/plasma ratios were considerably less, 0.29 [7, 8], than the ratio in this woman, which was 1.32. The discrepancy between these figures might be explained at least in part by interindividual differences in lipid contents and pH of the milk. Also, if only fore milk samples are being used a lower concentration will be present. However, in this study even higher milk concentrations of fluvoxamine might have been measured if also hind milk had been used since lipid soluble drugs such as fluvoxamine tend to concentrate in hind milk, which contains more fat than fore milk. Thus, using hind milk in this study would probably have resulted in an even higher M/P-ratio and estimate of the relative weight adjusted dose to the infant. Moreover, by using single pairs of samples the result will be less robust compared to studies with the design used in this study. Although no correlation between fluvoxamine clearance and systemic caffeine clearance was found in one study [9] the enzyme CYP1A2 has been proposed to be the major enzyme responsible for the metabolism of fluvoxamine [10]. This enzyme is not mature until 4 months of age [11] and in newborns even a low exposure of CYP1A2 substrate might therefore constitute a possible risk for adverse drug reactions. Due to ethical reasons we did not obtain blood samples in this infant, but no adverse drug reaction or unusual behaviour were observed, as was reported for the two previously exposed infants [7, 8]. This study indicates that the relative dose of fluvoxamine to the suckling infant is low, but higher than previously calculated values for fluvoxamine [2]. Moreover, the estimated relative dose is close to the values reported for other SSRIs such as sertraline [2, 12, 13], and paroxetine [2, 14, 15], but somewhat less than values reported for fluoxetine [2, 5, 16] and citalopram [2]. Based on the findings in this case and two others [6, 7] treatment with fluvoxamine seems compatible with breastfeeding. This statement should, however, be regarded as preliminary due to insufficient information about the possible clinical effects from the rather small doses to the suckling infant through breast-feeding.