Ten female calves of the Black and White breed aged seven to eight days were studied before and after seven days of treatment with spironolactone at a dose rate of 1 mg kg bodyweight - 1 twice a day. The treatment was associated with a statistically significant decrease in the half-life of antipyrine in plasma and an increase in its rate of metabolic clearance. The excretion of 4-hydroxyantipyrine in urine was significantly increased after the treatment with spironolactone, but there were no changes in the rates of excretion of 3-hydroxymethylantipyrine or norantipyrine. These results indicate that spironolactone enhanced the activity of the microsomal drug-metabolising enzyme system and induced cytochrome P-450 in the calves.