The effects of diazepam on the pharmacokinetics and pharmacodynamics of two lipophilic beta-adrenoceptor antagonists (propranolol and metoprolol) and a hydrophilic beta-adrenoceptor antagonist (atenolol) were compared in 12 subjects. Administration of propranolol and metoprolol produced small increases in the AUC0-8h for diazepam compared with placebo (P less than 0.05 for metoprolol). Atenolol had no significant effect on the AUC0-8h for diazepam. The increase in the AUC0-8h was accompanied by increases in the plasma concentrations of N-desmethyldiazepam. Diazepam had no significant effect on the pharmacokinetics of either propranolol or atenolol. The pharmacokinetic interaction could be attributed to inhibition of diazepam metabolism by the lipophilic beta-adrenoceptor antagonists. The pharmacodynamic studies showed that when compared with placebo or atenolol there was a significant impairment of kinetic visual acuity (KVA) when diazepam was co-administered with metoprolol. Although there was a significant correlation (P less than 0.02) between plasma concentrations of diazepam and impairment of KVA, the pharmacodynamic interactions may not be due solely to the small pharmacokinetic interaction observed.