Secretion of p-aminohippurate (PAH) from blood into the sperm-free lumen of the rat cauda epididymidis was studied by a stop-flow luminal perfusion technique. After an intravenous injection of [3H]PAH, radioactive material with chromatographic properties similar to those of free PAH appeared in perfusates. The highest concentration of PAH reached in the lumen was about 3 times that of the free PAH in blood. This was achieved after equilibration of the epididymis with perfusate for 20 min. Luminal accumulation of PAH by the epididymis was inhibited by intravenous injection of probenecid (32 mg/kg). However, deprivation of androgens by castration for 21 days had no effect on secretion. The rate of secretion (from blood to lumen) of PAH altered in a non-linear fashion with the concentration of free PAH in the blood and showed saturation at a free blood concentration of about 4 mumol/l. The apparent Km and Vmax values were 2 mumol PAH/l blood and 0.2 mumol PAH/h. 20 cm tubule, respectively. Based on the concentration profile of PAH across epididymal cells, a model was proposed for the secretion of PAH. It is concluded that the secretory mechanism is similar to that of the renal tubule. The significance of these results was discussed in relation to the functions of spermatozoa in the male reproductive tract.