In this study, the kinetics of pyrimethamine elimination via the urine was investigated. The experiments were carried out on six healthy male volunteers aged 23-32 years. The drug was administered orally (p.o.) in a single dose at three different concentrations i.e.: 50, 75 and 100 mg. The concentration of the drug in the urine was determined via the modified method of Bonini et al. and Garber et al. It was found that 13.4 +/- 1.3% of the dose eliminated via the urine was in unchanged form. The process of pyrimethamine elimination may be described according to an open kinetic two-compartmental model: the formula showing the course of pyrimethamine elimination over time has been given. Several examples of the quantitative exposure test have been proposed, which allow the calculation of the drug dose absorbed and thus the degree of toxicity to be determined. This test can also be useful in a controlled clinical setting.