The object of this study was to see if the blood level of 5-hydroxytryptamine (5-HT) increased under acute hypoxic stress in human beings and rats. Soldiers who had been stationed at high altititudes 2-7 months prior to study were selected. 17 had suffered from pulmonary oedema and the remaining eight, who had not developed the condition, acted as controls. The technique of YUWILER [18] with suitable modifications was used for estimation of 5-HT. No significant change in 5-HT levels was observed either in subjects or controls. Rats exposed to simulated high altitude for 6 h also did not show any significant change in blood 5-HT levels.