The pharmacokinetic analysis of plasma concentration-time curves after a single i.v. dose of 20 mg/kg sulphatroxazole (STZ) to calves and cows revealed a small distribution volume of STZ (mean VD(area)= 0.22–0.26 1/kg) and an age dependent elimination (mean t1/2 6.6–18.8 h). In calves and cows, STZ was extensively metabolized into the N4-acetyl and 5-hydroxy derivatives. In the plasma of calves, the N4-acetyl metabolite (N4-STZ) was present in greater amounts than the hydroxy metabolite (5-OH-STZ), while in cows' plasma concentrations of these two metabolites were similar. In the milk of dairy cows STZ concentrations paralleled those of the metabolites and were approximately 21 times lower than corresponding plasma concentrations. The mean plasma protein binding of STZ and its metabolites ranged from 36.4 to 82.5% of total concentration. The N4-STZ derivative was excreted by tubular secretion; the 5-OH-STZ and the parent compound, mainly by glomerular filtration. In calves the majority of STZ administered was excreted as N4-STZ (40–52%), while in cows the parent drug dominated the urinary excretion (36%).