Disposition kinetics and dosage regimen of sulphamethazine were investigated in sheep following single intravenous administration (100 mg/kg). The distribution and elimination half-lives were 0.15 +/- 0.04 and 4.5 +/- 0.3 h, respectively. The volume of distribution and total body clearance were calculated to be 0.6 +/- 0.1 l/kg and 90 +/- 13 ml/kg/h, respectively. Sulphamethazine bound with plasma protein to the extent of 76.6 +/- 5.6%. The calculated values of beta i and K beta were found to be 11.8 x 10(-6) mol/g and 1.93 x 10(-4) mol, respectively. A satisfactory intravenous dosage regimen of sulphamethazine in sheep would be 93 mg/kg followed by 79 mg/kg at 12 h.