The composition of epididymal plasma has been described in the boar (Mann, 1964; Crabo, 1965), the bull (Crabo, 1965; Wales, Wallace & White, 1966), the rabbit (Jones & Glover, 1973) and the ram (Scott, Wales, Wallace & White, 1963). In the rat, information on the subject is limited to measurements of ion concentrations (Levine & Marsh, 1971) in fluid collected by micropuncture techniques and to scattered values obtained from post-mortem material (Conchie, Findlay & Levvy, 1959; Dawson & Rowlands, 1959; Marquis & Fritz, 1965; Brooks, Hamilton & Mallek, 1974). The composition of this fluid in any species is important in relation to sperm survival and would seem to be of particular interest in rodents due to its relatively viscous consistency. It would also seem preferable to obtain such fluid from living animals, and with a minimum of contamination. The present paper, therefore, reports on the chemical composition of epididymal plasma obtained from the intact cauda epididymidis of anaesthetized rats. Twelve mature male rats (250 to 350 g) ofthe Wistar strain were anaesthetized with urethane (14% w/v in water; 10\m=.\0 ml/kg intraperitoneally), and the testes were exposed through a scrotal incision. A 23-gauge needle attached to a length of polyethylene tubing (1 mm o.d., 0\ m=. \ 5 mm i.d.) was inserted retrogradely into the ductus deferens, the other end of the tubing being attached to another needle and a syringe. The contents of the cauda epididymidis were collected by aspiration from both ductus deferentes aided by mild electrical stimulation (0\ m=. \ 5 to 5\ m=. \ 0 V) of the smooth muscle of the cauda. Because of the viscous nature of the epididymal fluid in the rat, the caudal contents were expelled gradually over a period of 1 to 2 min and between 30 and 50 \ g=m\ 1 of uncontaminated fluid were obtained.