Circadian variations of sulfamethoxazole pharmacokinetics were studied after a single oral administration of sulfamethoxazole, 50 mg/kg, to rabbits at 09:00 (a.m.) and 22 :00 (p.m.). The profiles of plasma sulfamethoxazole concentration showed from 6h to 24 h significant statistical difference (p<0.05) between 09:00 and 22:00. The half-life $t_{1/2} was significantly shorter in the morning (11.2 $pm$3.2 h) when compared to the nighttime (15.4 $pm$ 3.5h) (7< 0.05). The AUC was significantly decreased in the morning (1325 $pm$ 264${u}g/ml$.h) than that in the nighttime (2059 $pm$ 379${u}g/ml$. h) (p<0.05). Tota1 body clearance ($Cl_t$ was significantly higher when sulfamethoxazole was given in the morning (6.65 $pm$ 0.23 ml/min) versus in the nighttime (4.28 $pm$ 0.20 ml/min) (p<0.05).