5-Hydroxytryptamine (5-HT) blood levels were determined in men and pre-menopausic women born in Lima, Huancayo and Cerro de Pasco, situated at altitudes of 150, 3, 200 and 4, 338 m., respectively. Furthermore, post menopausic women, only from Lima, were subject of this study. In Lima 5-HT blood levels were higher in pre-menopausic women than those obtained in men and post-menopausic women. At high altitudes there were no sex differences in 5-HT blood concentration. However, men showed a higher concentration than those from Lima. Women from Lima, Huancayo and Cerro de Pasco showed similar 5-HT levels. Men and post-menopausic women secrete less estradiol than pre-menopausic women and on the other hand estrogens increase the uptake of 5-HT by platelets in vitro, our findings reflect that endogenous variations of estrogens may also affect the uptake of 5-HT in platelets in vivo. In men at high altitudes this or other mechanism may function to produce a higher 5-HT concentration in whole blood than those obtained at sea level. According to the similarity between blood platelets and tryptaminergic neurons, 5-HT variations in blood appear to reflect changes of the neurotransmitter level into the synaptic cleft, and could explain alterations in menopause and during exposure to high altitudes. © 1980, All rights reserved.