In Vivo Sodium, Potassium, and Sperm Concentrations in the Rat Epididymis**Supported by National Institutes of Health Research Grant 5RO1 HD09490.

In vivo samples of epididymal fluids were obtained through the use of micropuncture techniques. Microsamples from four areas of the rat epididymis were analyzed for Na+and K+concentrations and for sperm density. Na+values declined significantly from caput to corpus epididymidis (P<0.01), while K+and sperm concentrations increased significantly (P<0.01). A large water loss from the epididymal lumen was calculated, as well as net losses of both cations. Water losses may be explained on the basis of an active Na+pump; however, the effect of the absolute values of epididymal Na+and K+concentrations on sperm motility and fertility remains unresolved.