Angiotensin converting enzyme (ACE) is present in high concentration in the sperm cells of the rabbit. The enzyme is released during in vitro capacitation of the spermatozoa. It has been purified from the released proteins in the minimum capacitation medium after in vitro capacitation of the spermatozoa and is found to exist in two forms. One is of higher molecular weight and has low enzyme activity but the other one is of low molecular weight and has higher activity. Conversion of the higher molecular weight form to the lower molecular weight my be taking place during the capacitation of the spermatozoa.