Blood platelets have been suggested as amodel for serotonin uptake and storage in the brain. To examine possible effects of se- steroids, platelets from human volunteers were incugated for 15 min with oestradiol, testosterone, and dihydrotestosterone, and then incubated fro 1 min with radioactive serotonin. The effects of the various steroids on serotonin uptake were measured. At 10(-3) M, oestradiol decreased serotonin uptake by 70%, testosterone by 49%, and dihydrotestosterone by 25%. Both oestradiol and testosterone showed maximal enhancement of uptake at 10(-6) M; oestradiol increased serotonin uptake by 41%, and testosterone by 60%. Dihydrotestosterone maximally facilitated uptake by 40% at a concentration of 10(-4) M. No sex differences were found in either direction or in magnitude of change in uptake due to any hormone. It is suggested that the effects of oestradiol are of physiological significance and that the site of action of the sterioid effect on serotonin uptake is the cell membrane of the platelets.