Ten young volunteers (mean age 20 years) and ten elderly volunteers (mean age 89 years), including equal numbers of healthy men and women, were tested regarding adrenaline-induced platelet aggregation in vitro, platelet cyclic AMP content before and after beta-adrenoceptor stimulation with isoprenaline, as well as binding by platelet membranes of 125I-hydroxybenzylpindolol. In the aged patients there was a highly significant decrease in the concentration of adrenaline needed to produce irreversible platelet aggregation, as compared with the young. Platelet basal cyclic AMP content did not differ but the response to isoprenaline stimulation, expressed as the percentage rise of cyclic AMP content above the unstimulated level, was significantly decreased in the old subjects. The beta-adrenoceptor number was unchanged but the affinity decreased significantly in the old group. The data suggest that the increased aggregation response to adrenaline in old people could be due in part to a diminished functional capacity of the platelet beta-adrenoceptor.