MUN Research Unit, Medical School, Memorial University of Newfoundland, St John's, Newfoundland, AIA 1E5, Canada (Received 22 July 1975) Acidic steroids are the major progesterone metabolites excreted in rabbit urine (Cooke, Rogers & Thomas, 1963) and are C-21-carboxy compounds (Allen, Cooke & Thomas, 1968; Senciall & Dey, 1975). Resolution from steroid conjugates was obtained by alumina column chromatography (Senciall, 1973), and a major derivative, 3,6,20-trioxo-5α-pregnane-21-methyl ester, was identified by g.l.c. after oxidation and methylation (Senciall & Dey, 1975). Pregnancy and non-pregnancy urine samples of several species have now been screened for acidic metabolites of progesterone. [4-14C] Progesterone (52·8 mCi/mmol) was administered to female non-pregnant rabbits (i.v.; 5 μCi); pigs (i.m.; 10 μCi) and women (oral; 5 μCi);and [21-14C] progesterone (55·6 mCi/mmol) i.m. to guinea-pigs (5 μCi) and rats (5 μCi). Urine collected for 24 h contained means of 24·9; 25·9; 30·5; 15·9 and 11·8% of the doses respectively. Table 1