Few of the numerous synthetic gestagens now available resemble progesterone closely in structure and, with one exception, none is metabolized like progesterone. The 'pregnancy-maintaining' action of progesterone itself in the human female is open to question and the therapeutic use of synthetic gestagens in recurrent abortion has been disappointing (Shearman & Garrett, 1963). Progesterone cyclopentyl enol ether, almost the only true progesterone derivative among the synthetic gestagens, appears to be unique in being the only orally active compound which is metabolized to 5β-pregnane-3α:20α-diol in the body (Ercoli & Klopper, 1962). It therefore seemed of interest to compare the fate of this compound (Enol Luteovis, kindly supplied by Vister, Italy) with that of the parent compound, progesterone. Steroids were given to five normal male volunteers and urine collections made for periods up to 5 days after administration. Pregnanediol in urine was measured by the method of Klopper, Michie & Brown (1955).