Ram "rete testis" fluid (RTF) routinely collected throughout the year has been used as a source of inhibin. The mean flow rate and mean concentration of spermatozoa in the fluid remained constant during the first 12 days of cannulation. More than 50 castrated or cryptorchid rams have been treated with low doses of steroid-free RTF over a 25-h blood sampling period. Human serum albumin was injected as a control. RTF depressed both FSH and LH plasma levels although the pattern was different for each hormone. There was no change in prolactin secretion. LH secretion was affected first while FSH remained unchanged in castrated and in cryptorchid rams. Thereafter, the maximum depression of FSH plasma levels occurred at a time when LH started to return or had returned to preinjection levels in the cryptorchid and castrated animals respectively. In the cryptorchid rams, RTF suppressed pulsatile LH secretion which was present before treatment but in the castrated animals, RTF lowered LH plasma levels which were constant and showed no pulsatile changes before treatment. Both FSH and LH inhibitory activities have been found in all active fractions obtained by purification of RTF. These activities are papain-sensitive and active fractions have a high apparent molecular weight (greater than or equal to 100 000) as shown by gel filtration and ultrafiltration. These and other results in the literature have lead to a re-definition of inhibin as a protein factor of gonadal origin able to depress plasma levels of FSH and LH, even at low doses.