The present study was designed to understand the metabolism of 5-hydroxytryptamine (serotonin, 5HT), a dense granule constituent of platelets, in acidified platelet concentrates (PC) during in vitro storage in polyvinylchloride plastic containers at 22 degrees C. High-performance liquid chromatographic analysis showed that significant amounts of 5HT and its metabolites, 5-hydroxytryptophol and 5-hydroxyindole-3-acetic acid (5HIAA), were detected in the plasma of all PC stored for 3 days with a pH below 6.0, measured at 37 degrees C. PC with a platelet number over 7 x 10(10) cells maintained low PO2 values. Rapid fall in pH occurred in these PC with low PO2 values. The rate of lactate production of platelets increased several fold because of an insufficient oxygen supply to them. In PC with a sufficient supply of oxygen, the adenosine triphosphate (ATP) production rates were estimated to be 8 nmol/min/10(9) platelets. In PC acidified below pH 6.0, the oxidative phosphorylation nearly ceased, and glycolysis was also inhibited: ATP production rates averaged 1 nmol/min/10(9) platelets at 3-day storage. Numerous clusters of platelet aggregates were found in acidified PC, and platelets changed into spherical shapes with accompanying swelling. These platelets did not respond to adenosine triphosphate and failed to produce shrinking when suspended in hypotonic solutions. 5HT was not metabolized in acidified platelet-poor plasma. These data suggest that the metabolic activity of platelets was impaired when the pH of the PC fell below 6.0, leading to the release of platelet 5HT into the plasma. A substantial amount of 5HT might be metabolized by platelet monoamine oxidase into 5HIAA and 5-hydroxytryptophol.