Physiological characteristics of platelet serotonin (5-HT) levels in rats of Wistar origin were investigated by use of a recently developed method. By comparison of populations of male and female rats (N = 281) similar unimodal frequency distributions, with a tendency to higher values in females (1.61 vs. 1.70 micrograms 5HT/mg platelet protein; p less than 0.01), were found. For a group of 55 animals, monitored twice for this parameter within a week interval, a remarkable intraindividual constancy in time, the mean difference between two determinations being 5.5%, was shown. No age-dependence could be demonstrated for platelet serotonin concentrations in 5- to 30-week-old rats, nor were there significant circadian or seasonal oscillations.