Testicular spermatozoa are transported from the testis through the ductuli efferentes into the caput epididymidis in rete testis fluid (RTF). This fluid was first collected for analysis from conscious rams (Voglmayr, Waites & Setchell, 1966) and then from conscious bulls (Voglmayr, Larson & White, 1970) and anaesthetized wallabies (Setchell, 1970) Setchell, 1974). The rate of appearance of labelled compounds in RTF following their infusion into the circulation has yielded information about the blood-testis barrier to non-steroid (Setchell, Voglmayr & Waites, 1969;Waites, Jones, Main & Cooper, 1973) and steroid molecules (Cooper & Waites, 1975). ' None of the previous studies involved primates, and the opportunity arose to collect and analyse RTF from monkeys during the course of another investi¬ gation (N. Einer-Jensen and G. M. H. Waites, in preparation).Seven monkeys (six Macaca mulatta, 7-2 to 10-4 kg; and one M. fascicularis, 5 kg) were anaesthetized with 0-3 to 0-6 ml phencyclidine hydrochloride (Sernylan: Biocentric Labs. Ine; 20 mg/ml intramuscularly). Supplementary anaesthetic doses of phencyclidine (intramuscularly) and sodium pentobarbitone (Nembutal : Abbotts ; 50 mg/ml intravenously) or halothane (Fluothane : I.C.I.) were given when required. With aseptic precautions, each testis was exposed through a 2-to 3-cm paramedial incision in the anterior aspect of the scrotum. Without removing the testis from the scrotum, the connective tissue attachments of the caput epididymidis were cut to allow it to be gently retracted to expose the ductuli efferentes. These were occluded close to the testicular surface by two 3/0 silk ligatures, taking care not to damage the nearby veins (Text- fig. 1). The tunica vaginalis and skin incisions were then closed.The monkeys were anaesthetized again 20 to 27 hr later. With aseptic 505