Several environmental contaminants, notably dibromochloropropane (Whorton et al. 1977) and kepone (Taylor et al. 1978; Cannon et al, 1978) have been implicated in sperm deficiencies among occupationally exposed males. These incidents emphasize the need for adequate testing of chemicals for effects on the male reproductive system. Although important in clinical diagnosis, the evaluation of sperm motility has not been used extensively as a tool in chemical toxicology, particularly in commonly used small laboratory species. Measurements of sperm motility parameters are complicated by several variables including sample manipulation, temperature, cell concentration, time factors and the method of quantitation. In the present study, a simple objective procedure to estimate the proportion of motile epididymal spermatozoa in the rat has been developed.