Abstract: Sulfamerazine (SMR) administration (i.p., 3 days) of different doses to adult male rats showed significant decrease in electron transport components and drug metabolizing enzymes. In longer duration study, 100 mg SMR kg-1 caused significant decreases in cytochrome P-450 and activities of aminopyrine N-demethylase and aniline hydroxylase. In inhibitory dose of 100 mg SMR kg-1 was selected for dosing young male, old male and adult female rats. Sulfamerazine administration to young, old male and female rats resulted in a significant decrease in electron transport component and drug metabolizing activities at 100 mg SMR kg-1 dose level. All other parameters were unchanged. Sulfamerazine resulted in uncompetitive type of inhibition (Ki = 3.0 mM) of aminopyrine N-demethylase in vitro. Sulfamerazine destructed the spectral and catalytic activity of cytochrome P-450. The studies suggest that SMR is a substrate of the mixed function oxidase system and inhibition is dependent on dosage, age and sex of the animals.