Sodium and potassium were measured in epididymal plasma at six different levels of the epididymal duct in individual bulls, and the sperm concentration was estimated at the same levels. The sperm concentration was very low in the rete testis and the proximal part of the caput, but increased continuously to a maximum at the proximal end of the corpus. It fluctuated thereafter, being lower in the distal corpus and slightly higher in the cauda. In the rete testis the sodium content of the plasma was high and the potassium content low. The concentration of sodium decreased in the caput and proximal part of the corpus, where the potassium concentration reached its maximum, thus indicating a selective resorption of sodium with the fluid. In the rest of the corpus the concentration of sodium and potassium remained unchanged but in the cauda the concentration of both decreased markedly.