Objective:To investigate the effects of prenatal exposure of male rats to hydroxy-progesterone hexanoate (HPH) on ultra-structures of testis.Methods: Ten male rat puppies b to dams treated with 10 mg/kg body weight of HPH on the 1st, 7th and 14th day after conception were allowed to grow for 90 d; sacrificed, their testes dissected and prepared for electron microscope examination.Results: The testes of all rats b to dams treated with HPH had degenerated germ cells that lost their attachment to surrounding sertoli cells (SCs) and the basement membrane. The germinal cells were abnormal with cytoplasmic cavitations. SCs had small invaginated nucleus with peripheral nucleolus, folded cell membrane and large lipid droplets. The cytoplasm of SCs had small cavities and the mitochondriae appeared distended and had lost their cristae. The interstitial spaces were wider with few leydig cells (LCs) that had elongated nucleus with indented nuclear membrane and three prominent nucleoli with dense scattered chromatin. LCs cytoplasm contained abundant lipid droplets, numerous mitochondriae and cytoplasmic vacuoles. The seminiferous tubules germinal epithelium suffered severe degree of degeneration and there were no stages of spermatogenesis.Conclusion:Prenatal exposure of male rats to HPH induces ultra-structural changes in the seminiferous tubules, SCs and LCs as well as it hampers spermatogenesis. Thus the future fecundity of males b to females treated with HPH during pregnancy will inevitably be affected.