The effects of temperature on the male tract have been examined in the rat, rabbit and hamster, as well as other species that include a naturally cryptorchid rodent, the degu, and an insectivore, the musk shrew. In principle, a small increase in the temperature of the testis does not destroy the germinal epithelium; however, it reduces testis weight and sperm production and brings a greater incidence of morphologically abnormal spermatids and spermatozoa. This demonstrates that the testis can be partially suppressed yet remain functional in the face of moderate elevation in its ambient temperature. Selective imposition of abdominal temperature on the epididymis alone does not suppress sperm maturation there, and bilaterally cryptepididymal males remain fertile for long periods. However, deep body temperature changes at least the ionic and protein composition of cauda fluid by virtue of effects on the cauda epithelium, and it eliminates the special ability of the cauda to store and prolong the life of spermatozoa. Additionally, deep body temperature also immediately curtails the storage capacity of the cauda epididymidis, an effect that is reflected in a reduced diameter and apparently length of that segment of the cauda as well as vas deferens, which contracts during orgasm to provide the bulk of the ejaculate. One consequence of this, notwithstanding a normal sperm production by the testis, is a much smaller number of spermatozoa in the first ejaculate, and an atypically steep decline in the number in subsequent ejaculates produced by cryptepididymal males. Further effects of deep body temperature on the epididymis are seen in significantly faster (rabbit) sperm transport through it, and a reduction in the time required for capacitation of (hamster) spermatozoa, in vitro and in vivo. Man's scrotal surface temperature is chronically elevated by several degrees in the clothed state. Although observations are lacking for human "control" populations, certain of the temperature-related phenomena described in animals are nevertheless suggested variably in different measurable functions of the human male reproductive tract. These include a relatively low number produced/gm of testis and a poorer quality of spermatozoa released from it, a rapid epididymal transport and minimally developed sperm storage system in the cauda epididymidis. Finally, the character of the ejaculated spermatozoa in several respects may imply an imminent state of capacitation. In all, this circumstantial evidence makes it seem possible that the human epididymis as well as the testis often exists in a state of temperature-induced partial suppression.