The fate of trimethoprim and sulfamethoxazole and the combination of both these agents was studied in a group of healthy pregnant women who were undergoing therapeutic abortion. Assays were performed on samples of blood, urine, amniotic fluid and fetal tissues, using a standardized protocol for the selection of patients, dose administration, sample collection and assay techniques. A comparative evaluation of kinetics to assess the maternal handling and the distribution of trimethoprim throughout the fetoplacental unit disclosed no significant difference in the concentration within fetal fluids and tissue compartments. On the other hand, the concentration of sulfamethoxazole was lower in fetal tissues than in fetal fluids when relative ratios to trimethoprim were compared. The implications of the difference in behaviour of pharmacokinetic and clinical points of view.