Uterine fluid from estrogen-primed immature or mature ovariectomized rats has sperminhibitory activity when combined with H2O2 and either iodide or thiocyanate ions as measured by a loss of motility and by a decrease in pyruvate oxidation. The uterine fluid can be replaced by lactoperoxidase or myeloperoxidase which suggests that the sperm-inhibitory effect of uterine fluid is due to its peroxidase content. The uterine fluidmediated sperm-inhibitory system is inhibited by catalase and by a number of lowmolecular-weight compounds such as azide, reduced glutathione, cysteine, ergothioneine, and ascorbic acid. Heat-stable, low-molecular-weight inhibitors are found in seminal plasma and, to a lesser degree, in uterine fluid.