Platelet aggregation and ATP release were simultaneously measured in platelet-rich plasma samples obtained from humans of various age. In subjects aged over 59 years, an increase was found in platelet sensitivity to ADP and collagen as well as elevated aggregation amplitudes 5 min after induction with low concentrations of the aggregation agents. At 1 mumol/l ADP, old persons had a higher incidence of a secondary aggregation wave. Platelets from old subjects also released more ATP in response to collagen stimulation. Since no age-related changes are commonly found in the platelet count, the observed increase in platelet sensitivity to aggregation agents cannot be regarded as a mere compensatory reaction. An attempt is made to relate altered platelet function to peculiarities of lipid metabolism associated with ageing.