Spermatozoa acquire the capacity for motility as they traverse the mammalian epididymis. The biochemical basis for this induction of motility is still largely unknown. Current theories are discussed and recent studies from the authors' laboratory are described which indicate that two separate processes are involved and that these act synergistically. These processes are an increase in the intrasperm content of cyclic AMP during epididymal transit combined with the binding of a specific forward-motility protein. A second increase in cyclic AMP and stimulation by calcium ion is likely involved in the expression of the acquired potential for motility at the time of ejaculation.