Tests of psychometric function were performed in young, normal volunteers taking several beta-adrenoceptor antagonists. With single doses of atenolol, a cardioselective hydrophilic beta-blocker, dose-dependent effects were apparent and were maximal at a dose of 200 mg. The lipophilic non-selective beta-blocker, propranolol, also produced significant impairment of psychomotor tests but these were inversely related to dose, the longest effects being at a dose of 40 mg but with little effect at 320 mg. Subsequently, a multisubject comparison of propranolol and atenolol confirmed these findings and showed the effects to be of the same order of magnitude as those produced by diazepam. Chronic administration of atenolol 100 mg, nadolol 80 mg and diazepam 5 mg daily for seven days showed some effects with all drugs during the test period; however, these were sporadic rather than persistent. Overall, beta-blockers do appear to have central effects in man which can be demonstrated by psychomotor tests. However, the relevance of these central effects to maintenance therapy and skilled performance is unclear.