A study was conducted to see the effect of 15- and 30-day corticosteroid therapy (prednisolone 0.25 mg/kg/day) on the metabolism of antipyrine and chloramphenicol in rabbits. Antipyrine and chloramphenicol were given to rabbits orally at doses of 20 and 50 mg/kg, respectively on days 0, 15 and 30 of prednisolone therapy. The half-life of antipyrine and chloramphenicol were significantly reduced (p less than 0.05) after 15 and 30 days of the corticosteroid therapy. It is concluded that this effect might be due to the induction of liver microsomal enzymes.