Male rats were exposed to 4,340 m altitude for 4 days. At this time histological alteration in the testes were observed. There were tubules with pyknotic primary spermatocytes and spermatids, necrosis of numerous cells, and sloughing of primary spermatocytes. The damaged areas were limited by normal tissue. Serum testosterone increased significantly after 4 days of exposure. Administration of an anti-serotoninergic compound, cyproheptadine (10 mg/kg body weight), prevented the high altitude-induced changes in both spermatogenesis and serum testosterone. These observations suggest that serotonin is involved in the testicular changes that occurred when rats are acutely exposed to high altitude. © 1990 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.