Information relating to the effect of steroids on the metabolism and motility of human spermatozoa is limited despite the increasing interest in the use of these hormones for the purpose of contraception. In the present study it was shown that progesterone, testosterone, and 11-beta-OH-progesterone significantly inhibit the CO2 production of human spermatozoa. Estrone was ineffective. Progesterone and testosterone, but not estrone, also significantly depressed O2 uptake. In contrast to CO2 production and O2 uptake, lactic acid production was not significantly affected by any of the steroids studied. The inability of estrone to inhibit sperm metabolism was not due to its lower solubility relative to the other steroids tested. The data indicate that the effective steroids inhibit sperm respiration, but not the sperm glycolytic pathway.