The kinetics of sulfadiazine and trimethoprim excretion was determined. The examinations were performed on 10 healthy people. The compounds were given orally once, in the form of pure substances, applying three different doses: 80, 240 and 720 mg of sulfadiazine and 20, 60 and 180 mg of trimethoprim. In order to estimate the contents of sulfadiazine in urine, Bratton-Marshall's method was applied while the colorimetric method according to Vachek and Kakac was used in estimating the contents of trimethoprim. This method was adapted by Klimowicz to determine trimethoprim in blood and it was modified in our laboratory. It was stated that about 44 per cent of the administered dose of sulfadiazine and about 56 per cent of trimethoprim were excreted with urine in the form of unchanged compounds. The excretion processes of both compounds can be described in the way accepted for one-compartment kinetic models. The value of the half-life of sulfadiazine excretion is 8.4 hours while that of trimethoprim is 6 hours. The constant of loss is 0.083 for sulfadiazine and 0.118 for trimethoprim. The given formulae allow to calculate the rate of excretion of sulfadiazine and trimethoprim with urine at any time after being administered. The tests proposed to calculate the accepted drug doses, which are based on measuring the excretion rate of the unchanged substances with urine, so that they may be applied in controlled therapy and in poison therapy.