The relationship between mean platelet volume (MPV) and platelet arachidonic acid metabolism was examined by studying the ability of human platelets of different size to incorporate and metabolize tritiated arachidonic acid ([3H]AA). Platelet phospholipids were labelled with [3H]AA and the platelets were then fractionated into size-dependent subpopulations by counterflow centrifugation. The incorporation of [3H]AA increased through the fractions proportional to the MPV. After thrombin stimulation the per cent of total 3H-radioactivity released from the platelets decreased as the MPV increased. However, fractionation of the released 3H-radioactivity by HPLC (high performance liquid chromatography) demonstrated that MPV had no significant influence on the per cent of total platelet 3H-radioactivity released as cyclooxygenase products or as HETE (12-hydroxyeicosatetraenoic acid) but that the release of unmetabolized [3H]AA decreased as MPV increased. In separate experiments using unlabelled platelets the absolute release of thromboxane B2 (TXB2) after collagen- and thrombin-induced aggregation was measured by radioimmunoassay and was found to increase in proportion to the MPV. These results demonstrate that the release of arachidonic acid metabolites is qualitatively similar in platelets of different size. However, the absolute ability of platelets to incorporate arachidonic acid, convert it to active metabolites and release them is proportional to their volume. The ability of platelets to release unmetabolized arachidonic acid varies inversely with their MPV.