Sperm maturation in the rat epididymis is dependent on the secretion of specific proteins by the epididymal epithelium and subsequent interaction of these proteins with spermatozoa. Evidence has shown that fertility and motility development of epididymal spermatozoa may be impaired by interfering the interaction of these proteins with spermatozoa. When the spermatozoa reach the cauda epididymidis, they are fully mature but their longevity is maintained by being stored in a quiescent state in the cauda. The unique ionic medium therein (low Na+, low Ca2+, high K+and low pH) suppresses sperm motility and hence reserving energy for the vital processes of capacitation and fertilization. During ejaculation, when the spermatozoa are mixed with the copious secretion from the accessory glands they burst into vigorous motility. This results from an influx of sodium coupled to efflux of K+and H+across the mature sperm membrane. In the presence of a peptide secreted by the cauda epididymidis, these ionic events activate the already mature but otherwise inactive spermatozoa to full motility.