Eight experiments were carried out on eight clinically healthy non-pregnant ewes. Each animal was injected intravenously with either sulphadiazine or sulphadimidine at a dose rate of 100 mg/kg body weight. A two-compartment pharmacokinetic model was developed to describe the disposition of these drugs. The elimination half-lives were 7.15 +/- 0.58 h and 9.51 +/- 0.59 h and the distribution half-lives were 0.56 +/- 0.07 h and 0.42 +/- 0.05 h for sulphadiazine and sulphadimidine, respectively. The apparent specific volumes of distribution were less than 1 litre/kg (0.410 and 0.501 litres/kg for sulphadiazine and sulphadimidine, respectively) which indicates a relatively lower distribution of these drugs to tissues than in plasma in sheep. The degree of plasma protein binding was similar for both drugs (19.15 +/- 0.55% and 23.12 +/- 0.32%) for sulphadiazine and sulphadimidine, respectively). Serum concentrations of ketone bodies, total lipids and calcium were significantly reduced, and blood glucose concentration significantly increased following administration of both of these sulphonamides, whilst serum total protein concentration was unaltered. The serum cholesterol concentration was significantly reduced following sulphadiazine administration, but not after sulphadimidine