A sensitive microbore high-performance liquid chromatographic method with electrochemical detection has been developed for the measurement of small quantities of serotonin (5-hydroxytryptamine; 5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in rabbit platelets. The limit of detection of 5-HT is 0.1 ng/ml. To evaluate the 5-HT release from platelet suspension, the aggregating agent collagen was added at concentrations of 10, 30 and 100 micrograms/ml and the 5-HT concentrations rose from the base level (11.80 +/- 0.42 ng/ml) to 123.97 +/- 11.02, 361.96 +/- 17.90 and 470.45 +/- 35.46 ng/ml, respectively. Further results demonstrated that aspirin inhibits collagen-induced 5-HT release from platelet suspension significantly.