SummaryWhilst in their native epididymal fluid, sperm from the caput epididymis of the rat and hamster contain significantly (P<0.01) greater amounts of cAMP than do sperm from the cauda epididymis. The cAMP levels in both cell types from these species underwent a rapid increase concomitant with dilution to a density of 20 × 106/ml. Further analyses in the hamster indicated that this increase was calcium‐dependent, and could be enhanced by treatment with the calmodulin antagonist, calmidazolium. Dilution of hamster sperm to a concentration of 1 × 106/ml was not associated with a rapid rise in cAMP levels. This effect was shown to be due to the dilution of a component in epididymal plasma. When incubated at this lower density, the cAMP content of hamster caput sperm remained low over a 3 h period, whilst similarly treated caudal sperm exhibited a progressive rise in cAMP levels. Thus, in contrast to other species, cAMP does not appear to play a pivotal role in acquisition of the capacity for movement during epididymal maturation in the rat and hamster. However, this nucleotide may be involved in the post‐ejaculatory modifications of motility which accompany the terminal stages of capacit