AbstractA review is presented of the effects of steroids having hormone‐like activity upon the following reproductive processes in mammals: (a) spermatogenesis, (b) ovulation, (c) fertilization, (d) oviducal and uterine travel of the free ovum, and (e) blastocyst growth and implantation. Description is given of a number of natural and synthetic steroids which inhibit spermatogenesis and ovulation. By and large, estrogens are especially effective in the former and progestins in the latter, but aparently some species differences in response to potent substances do exist. Progestins have been shown to inhibit fertilization in experimental animals either through an effect on sperm travel or on sperm capacitation. The rapid expulsion of eggs from the fallopian tubes is accomplished by a variety of synthetic and natural estrogens upon administration in significant dosage in test animals. Decidualization and other processes related to implantation are also steroid labil