The plexi of the male reproductive tract have components of both the autonomic and sensory nervous systems. Rat epididymis was found to be a rich source of substance P. Substance P levels in the epididymis were higher by about 2.8 and 19.3 times than those in the prostate and seminal vesicles, respectively. Seminal vesicles were found to be a rich source of enkephalins. They had about 2.9 and 2.6 times higher leucine enkephalin levels than epididymis and prostate, respectively. Human seminal plasma contained about 47 times higher levels of leucine enkephalin than substance P. Using the split ejaculate technique, it has been demonstrated that early fractions of the human ejaculate contain fluids from prostate (and possibly epididymis), whereas later fractions represent seminal vesicle secretions. A low exogenous concentration of substance P (400 nM) increased sperm motility, whereas leucine enkephalin (100 microM) depressed it. Substance P (1-10 micrograms/mL) and muscarinic agonists enhanced the adrenergic transmission of the rat vas deferens to electrical stimulation. Leucine enkephalin (1-10 micrograms/mL) depressed adrenergic transmission and antagonized the effects of substance P and muscarinic agonists. These studies suggest that substance P-like tachykinins may play a role in sperm maturation, in expulsion of fluid from the epididymis, and in initiation of motility, whereas leucine enkephalin-like peptides may contribute to the orgasmic experience and detumescence.