A simple and reliable method for individual monitoring of platelet serotonin in rats is developed. Platelet-rich plasma is prepared under standardized conditions from 1 mL of venous blood and the platelets are quantitatively separated by a highly reproducible procedure. Platelet serotonin content is determined spectrophotofluorometrically and the results are comparatively expressed per standardized platelet rich plasma sample (1.01 +/- 0.18 microgram), per mg of platelet protein (1.57 +/- 0.15 microgram) and per 10(9) platelets (2.16 +/- 0.38 micrograms). Normal distribution of platelet serotonin levels in a sample of 338 animals is shown. By use of the described method, the intraindividual stability of platelet serotonin concentration in rats is demonstrated for the first time.