Abstract THE sex of mammals has been altered by treating neonatal animals with several steroid sex hormones. There is one report that sexes of birds were altered during a similar stage of development after hatching (Wentworth, 1968). Mazer and Mazer (1939), Bradbury (1947), and Huffman (1941) observed that injection of prepubertal female rats with androgen results in subsequent infertility. Five-day-old rats receiving a single injection of testosterone propionate were permanently sterilized, but injection of testosterone to 20-day-old mice had no effect on fecundity (Barraclough and Leathem, 1954). In general, investigators studying female mice, rats and hamsters have noted persistent cornification of the vaginal epithelium, abnormal sexual behavior and absence of corpora lutea after treating neonatals with testosterone (Barraclough, 1961; Gorski and Barraclough, 1963; Swanson and van der Werf ten Bosch, 1965; and Swanson, 1966). In recent work with mammals, sexual maturation of either sex has been effected after treating neonatals with…