In eight young healthy men the effects of short-term treatment with prenalterol, a new beta-adrenergic agonist, on peripheral hemodynamics and leg muscle metabolism were studied at rest and during exercise at a moderate and a heavy workload. At rest prenalterol treatment significantly increased pulmonary oxygen uptake, heart rate, rate pressure product, and leg blood flow and decreased leg arteriovenous oxygen content difference. During exercise fewer hemodynamic effects of prenalterol were observed. At the moderate workload there was a slight increase in heart rate, but at the heavy workload heart rate fell. Prenalterol raised the arterial concentrations of free fatty acids and glycerol at rest by 70% and 40%, but during exercise these alterations were not present. The plasma levels of epinephrine and dopamine in subjects at rest decreased significantly during influence of prenalterol, but these differences were also abolished during exercise. The leg exchange of metabolites and the concentrations of endogenous muscle metabolites were not influenced by prenalterol, although the lactate concentrations in muscle tended to be lower at rest and during exercise. The results at rest demonstrate that prenalterol in this situation acts predominantly as a stimulator of beta 1-adrenoceptors. During heavy exercise, however, when the endogenous sympathetic tone is high, the beta-stimulating effects are no longer overt and instead reduced heart rate indicates a blocking effect on beta-adrenoceptors. The hemodynamic and metabolic results are concordant with the hypothesis that prenalterol acts as a partial beta-receptor agonist.