Rat spermatozoa are immotile in the cauda epididymidis and are kept quiescent by a protein which increases viscoelasticity of cauda luminal fluid. How species-specific this phenomenon is, is unknown. In the present study, the motility of cauda epididymal spermatozoa of rats, hamsters, guinea pigs, rabbits and humans have been investigated. Sperm motility was observed in undiluted cauda sperm samples and in samples diluted with physiological diluents with or without Ca++, among others. Hamster sperm were studied in further detail to determine if the motility inhibiting factor in hamster cauda lumen fluid had characteristics similar to those previously described in the rat. Cauda fluid protein concentrations and apparent viscoelasticity were also determined and related to cauda sperm motility in all species. The results demonstrated that all species studied except rabbits have immotile sperm in their native cauda fluid and that additional Ca++ is not a factor in the initiation of motility. Cauda sperm immotility is not always related to fluid viscosity, however, so other as yet unknown mechanisms must be called upon in some species. The vigorous motility of rabbit spermatozoa in their native fluid implies that a fundamental difference exists in the relationship between epididymis and spermatozoa in rabbits from that observed in other species.