SUMMARY1. A microperfusion technique has been developed to study transport processes in the rat cauda epididymidis in vivo.2. Na+ and water were found to be reabsorbed by the perfused rat cauda epididymidis at the rates of 3-000 + 0-25 n-equiv cm-' min-(mean + s.E., n = 14) and 20-7 + 1-7 nl. cm-1 min-' (mean + s.E., n = 14) respectively. Reabsorption of Na+ was isotonic.3. K+ was found to be secreted into the ductal lumen at the rate of 0-124 + 0-016 n-equiv cm-1 min' (mean + S.E., n = 14).4. Na+ reabsorption and water reabsorption were abolished by removing Na+ ions from the perfusion medium. The dependence of rate of net Na+ reabsorption on the intraluminal Na+ ion concentration showed saturation kinetics, with the apparent K. values of about 20 mm Na+. The dependence of water reabsorption on the intraluminal Na+ ion concentration also followed closely that of Na+.5. Application of amiloride 10-4 M) to the perfusion fluid abolished both Na+ and water reabsorption by the rat cauda epididymidis.6. Removal of chloride from the perfusion fluid had no effect on Na+ and water reabsorption but increased the K+ secretion rate by threefold.7. Proteins were also found to be secreted by the rat cauda epididymidis at a rate of 11-7+ 1-8 ng cm-1 min-(mean+s.E., n = 11). The secretary rate was not dependent on the intraluminal Na+ ion concentration. 8. Castration in rats abolished the reabsorption of Na+ and water and secretion of K+ and proteins by the rat cauda epididymidis. These effects could be reversed by injecting testosterone propionate into castrated rats.9. The possible role of these transport processes in sperm maturation is discussed.

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A technique is described fro measurement of rate of fluid reabsorption in a segment of the rat cauda epididymidis in vitro. The basal rate was 2-63 +/- 0-22 mul/cm2/30 min and was dependent on temperature and inhibited by 2,4-dinitrophenol, suggesting that fluid reabsorption is an energy-dependent process. About 50% of the fluid reabsorption was dependent on intraluminal sodium ions. This Na+-dependent component was inhibited by addition of amiloride to the intraluminal fluid and ouabain to the peritubular fluid.

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