A sensitive and reproducible reversed-phase high-performance liquid chromatography with electrochemical detection for determination of serotonin in human plasma is described. The method has an average coefficient of variation of 3.5%; the recovery of serotonin and 5-hydroxy-N-methyltryptamine (internal standard) accounts for 95 +/- 2%. Average serotonin concentrations in platelet-free plasma and in platelet-rich plasma from 21 normal subjects 22-63-year-old) ranged from 0.6 to 4.9 ng/ml (mean 2.8 +/- 1.3 SD) and from 38.3 to 106.5 ng/10(8) platelets (mean 66.8 +/- 18.9 SD), respectively. The release of endogenous serotonin from human aggregating platelets challenged by collagen or adenosine diphosphate has been quantitated.