The motility and metabolism of hamster epididymal spermatozoa in the presence of secretions from the ventral prostate gland have been studied. The aerobic metabolism and motility of hamster epididymal spermatozoa were depressed by a factor of large molecular size which could be partially but not completely destroyed by heating, even at 90° C for 10 min. Treatment of the ventral prostate secretion with EDTA had no effect on this factor, indicating that its activity is not dependent on divalent cations. The presence of 0·01 to 0·1% cysteine had no visible effect on the activity of this factor, suggesting that sulphydryl binding is also not involved. Its adverse effect on spermatozoa could not be neutralized by the secretion of other accessory glands. Hamster epididymal spermatozoa were more sensitive to the hamster ventral prostate secretion than mouse and rat epididymal spermatozoa and the ventral prostate secretions of the mouse and rat appeared to have no adverse effect on their epididymal spermatozoa.