The phospholipids and their fatty acids of the inner and outer plasma membrane leaflets of the maturing goat caput-, corpus- and cauda-epididymal spermatozoa were analyzed by treating the intact spermatozoa with phospholipase C and trinitrobenzene sulphonate. The inner and outer membrane showed marked differences in the phospholipid composition at all stages of epididymal sperm maturation. The outer membrane was rich in phosphatidylcholine (PC) and sphingomyelin (SPH) whereas the inner leaflet was dominated by phosphatidylethanolamine (PE). Although the ratio of PE/PC in the inner membrane was similar in both the mature cauda sperm and the immature caput sperm, it decreased significantly in sperm undergoing maturation in the corpus-epididymis. The distribution of the saturated and unsaturated fatty acids in the phospholipid fractions of both the membrane leaflets underwent profound alterations during the epididymal maturation. The data demonstrate asymmetry of phospholipids and their fatty acids in the sperm inner and outer plasma membranes and this lipid asymmetry is greatly altered during epididymal maturity of the male gametes. © 1993.