Diltiazem and verapamil inhibit oxidative drug metabolism both in vivo and in vitro. We compared their effects on the stereoselective pharmacokinetics and protein binding of propranolol in 12 subjects. After 6 days of coadministration with racemic propranolol, diltiazem caused decreases of 27% and 24% in d-propranolol and 1-propranolol oral clearances, respectively (p less than 0.05 versus control). With verapamil, d-propranolol oral clearance decreased 32% (p less than 0.05), and 1-propranolol oral clearance decreased 26% (p less than 0.05). The unbound fraction of d-propranolol was higher than that of 1-propranolol (p less than 0.05), but the protein binding was not altered by diltiazem or verapamil. Both drugs therefore decreased the unbound oral clearance of each propranolol enantiomer (p less than 0.05). Verapamil caused a stereoselective effect and increased the d/l ratio of propranolol serum concentrations (p less than 0.05) and decreased the d/l ratio of oral clearance (p less than 0.05).