In vitro activity: Sulfamethoxypyridazine is found to be highly effective in a preliminary study of P. carinii-infected rats. Sulfamethoxypyridazine is also found to be as effective as sulfamethoxazole in P. carinii-infected rats at 1 mg/kg of body weight/day. Sulfamethoxypyridazine is effective aganist a murine model of P. carinii with ED50s of 0.06 mg/kg/day and 0.08 mg/kg/day as determined by the Giemsa and silver stain scores, respectively. Sulfamethoxypyridazine at either 0.1 mg/kg/day or 0.3 mg/kg/day is found to be significantly more effective than sulfamethoxazole at 0.47 mg/kg/day) aganist a murine model of P. carinii. Sulfamethoxypyridazine exhibits the biological half lives of 11.0 hours and 13.7 hours following im and sc administration, respectively, in goats. Sulfamethoxypyridazine exhibits the systemic availabilities of 68.6% and 58.7% following im and sc administration, respectively, in goats. Sulfamethoxypyridazine exhibits the distribution and elimination half life of 0.1 hour and 6.28 hours, respectively, in goats. Sulfamethoxypyridazine exhibits the values of apparent volume of distribution at steady state and total body clearance of 0.39 mL/kg/min and 0.73 mL/kg/min, respectively, in goats.