1. Changes in forearm blood flow to intra-arterial infusion of isoprenaline and the chronotropic response to intravenous boluses of isoprenaline were measured in 15 healthy volunteer subjects, eight younger than 25 years and seven older than 50 years. Intra-arterial blood pressure and basal plasma renin activity, adrenaline and noradrenaline were also measured. 2. Young subjects exhibited a greater increase in forearm blood flow than old subjects, to all four doses of isoprenaline used, a greater cardiac isoprenaline responsiveness (measured by the increase in heart rate; P less than 0.001) and a higher renin (P less than 0.02). 3. Resting values of blood pressure, forearm blood flow, adrenaline and noradrenaline were not significantly different in young and old subjects. In the latter, noradrenaline correlated with forearm blood flow (r = -0.77, P less than 0.05), forearm vascular resistance (r = 0.86, P less than 0.02) and mean arterial pressure (r = 0.83, P less than 0.02), whereas in the younger subjects forearm blood flow was related to adrenaline (r = 0.78, P less than 0.05). 4. These data provide evidence for an age-related parallel reduction in cardiac, peripheral vascular and renal beta-adrenoceptor-mediated responses.