1. Effects of the beta-adrenoceptor antagonists, propranolol (40, 80 and 160 mg) and atenolol (50 and 100 mg) on the electroencephalogram and on body sway, were studied in 12 healthy male subjects. The study was double-blind, and included two placebos and an active control, oxazepam (15 mg). Medication was ingested at 11.00 h, and assessments were made before, and at 2 h and 4 h after ingestion. 2. All doses of both beta-adrenoceptor antagonists modified the electroencephalogram, and the changes reported were statistically significant at probability levels of less than 5%. The circadian rise in alpha activity was reduced by both beta-adrenoceptor antagonists as well as by oxazepam. Atenolol also decreased beta activity. 3. Body sway was modified by atenolol and oxazepam (P less than 0.05). The increase with oxazepam was most marked in the low frequency component (0.05-2.25 Hz) of the spectrum, while atenolol modified only the component of higher frequency (2.25-4.0 Hz). 4. These observations suggest that propranolol and atenolol have a sedative effect, and that hydrophilic antagonists are unlikely to be free of central activity. The changes in body sway could imply that peripheral mechanisms may be modified at least with atenolol.