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 altitudes 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 level