The effect of clinically used equipotent doses of nonselective (beta 1/beta 2; propranolol) and selective (beta 1; atenolol) beta-adrenoceptor blockers on thermoregulation was studied during prolonged exercise in the heat. Oral propranolol (160 mg/day), atenolol (100 mg/day) or matching placebo were taken for 6 days each by 11 healthy young adult caucasian males. Subjects participated in 2 h of block-stepping at a work rate of 54 W in an environmental chamber with a temperature of 33.2 +/- 0.3 degree C dry bulb and 31.7 /+- 0.3 degree C wet bulb, 2 h after ingestion of the final dose of each drug. Both active agents produced similar marked (P less than 0.001) increases in subjective perception of effort, the mechanism of which was not immediately evident from changes in serum electrolytes, blood glucose, blood lactate, or ventilatory parameters. Propranolol did, however, cause a greater rise in serum K+ than placebo (P less than 0.02) and atenolol (P = NS) after exercise. Although rectal and mean skin temperatures were insignificantly altered by beta-adrenoceptor blockade, an increased total sweat production was noted with propranolol (P less than 0.01 vs. placebo) and to a lesser degree atenolol (P = NS vs. placebo) therapy. Analysis of the time course of sweat production showed the propranolol-mediated enhancement of sweating to ensue largely during the initial hour of block-stepping and to be transient in nature. The scientific and clinical implications of this observation will be dependent upon the precise underlying mechanism, a factor not identified by the present study.