The degrees of interactions between the H2-receptor antagonists, cimetidine and ranitidine, and several beta-adrenoceptor blockers were investigated in healthy volunteers following 7 days of oral monotherapy with penbutolol, propranolol, metoprolol, pindolol and atenolol, and after co-administration with each of the H2-receptor antagonists. The kinetic parameters of unmetabolised penbutolol and penbutolol glucuronide were unaffected, whereas the levels of 4-hydroxypenbutolol and 4-hydroxypenbutolol glucuronide were significantly reduced. Furthermore, cimetidine led to a marked increase in propranolol and metoprolol plasma levels. During co-administration with cimetidine, pindolol plasma levels were only slightly raised, whereas the pharmacokinetics of atenolol were not affected. With regard to pharmacodynamics, the inhibition of exercise-induced tachycardia by each of the beta-adrenoceptor blockers was not affected by cimetidine. Ranitidine did not alter atenolol plasma levels, but did raise the peak plasma concentration of metoprolol by about 30%. It is concluded that cimetidine interactions do occur and can be predicted for substances metabolised by the cytochrome P-450 pathway.