Administration (3 mg/kg body weight/day, for 21 days) of prostaglandin F2 alpha (PGF2 alpha) caused marked suppression of spermatogenesis and significant reduction in the weights of the testis, epididymis and accessory sex glands. The seminiferous tubules were devoid of spermatozoa and contained only Sertoli cells, spermatogonia, spermatocytes and occasionally spermatids; several multinucleated giant cells were observed in the lumen of the tubules. The Leydig cells were atrophied. The levels of RNA, DNA and protein in the testis were, however, unaffected by drug therapy. In drug-treated mice the epididymal epithelium presented a degenerate appearance; the lumen was generally devoid of spermatozoa and contained mainly exfoliated immature germ cells and sperm debris; cauda epididymidal spermatozoa, when present, were immotile and fragmented. PGF2 alpha treatment also caused significant decrease in the levels of sialic acid in the caput and cauda epididymides and of fructose in the seminal vesicle. The results suggest that the regressive changes induced by PGF2 alpha in the reproductive organs of the mouse are due to the interference with the secretion of androgen. The alterations induced in the reproductive organs by administration of PGF2 alpha were reversible and 56 days after drug withdrawal the organs returned to their normal state.