Enkephalins are one of the opioids present in human semen and to date their function in this tissue remains unknown. The present work studies enkephalin-degrading enzyme activities, puromycin-sensitive alanyl aminopeptidase (AAP-S), puromycin-insensitive alanyl aminopeptidase N (Ap N) and neprilysin (NEP) in human seminal fractions. AAP-S activity was not detected in any fractions, whereas Ap N appeared in soluble and particulate sperm fractions in seminal fluid and in prostasome fraction. With regard to NEP activity, this was exclusively located in prostasome membranes. The high activity values observed in the prostasome fraction suggested that these peptidases and their substrates could be involved in seminal physiology.