Carnitine and glycerylphosphorylcholine (GPC) levels have been measured in the reproductive tissues of male rats, with special reference to the epididymis. The increase in concentration of these compounds in reproductive tissues during the prepubertal and pubertal periods has been examined and their distribution along the epididymis has been determined in mature animals. Carnitine accumulates in the epididymal plasma in the cauda epididymidis whilst GPC accumulates in the caput epididymidis. By developing a new technique for the collection and separation of epididymal spermatozoa and undiluted fluid from the cauda epididymidis, the concentrations of carnitine, acetylcarnitine and GPC have been measured and the free fatty acid composition of epididymal plasma has also been determined and compared with blood plasma. By examining the effects of castration, testosterone replacement and cryptorchidism, the accumulation of carnitine and GPC in the epididymis has been shown to be under androgen control. By contrast, the anti-androgens, cyproterone acetate and medroxyprogesterone acetate, did not influence secretory activity, perhaps because their effect was nullified by a high androgen concentration in epididymal plasma. The antifertility compound \ g=a\ \ x=r eq-\ chlorohydrin was also shown to be without effect on the concentration of carnitine and GPC in the reproductive tract. The relationship of carnitine and GPC to the metabolism of fatty acids by spermatozoa is discussed.

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