The pharmacokinetic behaviour of sulphamethoxazole and trimethoprim was studied after combined intravenous (i.v.) administration at doses of 20 mg/kg and 4 mg/kg, respectively, and after oral administration at doses of 50 mg/kg and 10 mg/kg. The serum concentration versus time data after i.v. administration were best described by the biexponential equations C = 34.77.e-2.655.t + 39.03.e-0.241.t for sulphamethoxazole and C = 3.29.e-3.878.t + 0.83.e-0.306.t for trimethoprim. Mean biological half-lives of the drugs were 2.89 +/- 0.11 and 2.38 +/- 0.33 h, respectively. The distribution volumes (V area) were 0.475 +/- 0.026 l/kg (sulphamethoxazole) and 3.89 +/- 0.61 l/kg (trimethoprim). Orally administered sulphamethoxazole and trimethoprim were rapidly absorbed. The maximum serum concentrations were reached 0.5-1 h after administration. The bioavailability was 81% for sulphamethoxazole and 41% for trimethoprim.