Recent demonstrations that female reproductive physiology is highly responsive to environmental factors have led to a growing appreciation of the extent of variation in reproductive function and the adaptive nature of much of that variation. Male reproductive function has received much less attention from anthropologists, whether as an aspect of human variation, as a possible contributor to population variation in fertility, or as the product of selective pressures. Yet, an understanding of male reproductive function, along with that of females, is obviously important for a complete picture of human evolutionary ecology. We review our current understanding of male reproductive function, its response to environmental factors, and implications for male fecundity. Studies among Western populations indicate that male reproductive function is sensitive to environmental stresses, including weight loss, dietary composition, exercise, disease, and psychological stress. Indices of reproductive physiology among males in some non-Western populations are consistent with the notion that chronic environmental stresses can lead to suppression of testicular function. The specific causes of this suppression are not known, nor are the implications, if any, for male fecundity and fertility understood. Further research on males is called for, including the effects of hormonal changes on reproductive behavior and characterization of reproductive function in a wider range of populations. Evolutionary models of the selection pressures underlying male reproductive function should be helpful in focusing these investigations. © 1995 Wiley-Liss, Inc.