The heart rate response to isoprenaline in 11 subjects aged 19-46 years was compared with lymphocyte beta-adrenoceptor numbers and lymphocyte cyclic AMP responsiveness. The dose of isoprenaline required to increase heart rate by 25 beats/min ( CD25 ) increased as a function of age (r 0.79; P less than 0.01). Lymphocyte receptor numbers also correlated directly with age (r 0.61; P less than 0.05), but there was no true correlation between CD25 and lymphocyte receptor numbers. Baseline and maximum lymphocyte cyclic AMP concentrations in response to isoprenaline stimulation in vitro were unrelated to age, CD25 or receptor numbers. This study confirms previous findings of a reduced responsiveness with age and a rise in receptor numbers in young and middle-aged adults. However, the decline in the heart response to isoprenaline was unrelated to any measurable change in beta-adrenoceptor concentration or responses, at least in the isolated lymphocyte.