1. Cimetidine pretreatment of male Sprague-Dawley rats caused a significant increase in the specific content of total hepatic cytochrome P-450, supporting the hypothesis that this H2-receptor antagonist has monooxygenase induction effects. 2. Quantitative ultrastructural studies of liver of cimetidine-pretreated animals also supported this hypothesis in showing a significant proliferation of smooth endoplasmic reticulum. These ultrastructural changes were qualitatively similar to those produced by treatment of rats with phenobarbital, a well-characterized monooxygenase-inducing agent whose effects were studied for comparative purposes. 3. Competitive inhibition of metoprolol alpha-hydroxylation by cimetidine in liver microsomes prepared from untreated animals (Ki = 18.8 microM) was also demonstrated. 4. These results allowed testing of the hypothesis (Burnet et al. 1986) that inhibition of a defined monooxygenase should lead to induction of the synthesis of the relevant cytochrome P-450 isozyme. 5. The finding that metoprolol alpha-hydroxylase activity of liver microsomes was lowered, not elevated, by pretreatment of animals with cimetidine argues against the concept of a causal link between monooxygenase inhibition and induction.