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In the first part of this study the availability of folinic acid (FA) and its main active circulating metabolite, 5-methyltetrahydrofolate (5-MTHF), were studied in plasma and cerebrospinal fluid (CSF) from normal subjects after i.v. administration of 100 and 250 mg of FA. 5-MTHF rapidly appeared in plasma, the maximum value being reached at the first observation time point (1 h). FA was eliminated in plasma more slowly than 5-MTHF. Between the two doses, there was no evidence of modification in pharmacokinetic parameters (terminal half-life, clearance) for either FA or 5-MTHF in plasma and CSF; 5-MTHF was the only product detectable in CSF. Considering FA plus 5-MTHF together, the AUC (area under the curve) ratios between CSF and plasma were close to 1%. 5-MTHF was cleared very slowly from CSF (t 1/2 = 85 h). This finding suggested possible accumulation of 5-MTHF in CSF during repeated administration of FA combined with medium or high dose MTX. In the second part of the study, dealing with a group of eight children treated by such protocols, an increase in CSF 5-MTHF was detected from cycle to cycle in five (r = 0.91, P less than 0.01) with a maximum at 5 x 10(-8) M. This progressive accumulation of 5-MTHF in CSF may have a negative effect on the local action of MTX and should be taken into account for therapeutic strategies designed for the management of meningeal leukaemia.