PIP: A study confirming the in vivo D-homoannulation of an ethynylated steroid evidenced by its specific beta-stereochemistry involved the administration by sc injections of mestranol in 6 adult female guinea pigs. 17 alpha-ethinyl estradiol as a monoconjugate (glucuronide) and a diconjugate (glucuronide and presumably sulfate in urine), D-homoestradiol-17a beta and D-homoestrone were identified as urinary metabolites of mestranol in these guinea pigs. Very few metabolites retained the carbon-14-methyl ether. Chromatographic evidence was given for the presence of little mestranol with 11-tritiated-hydrogen to carbon-14 ratio. The large percentage of free metabolites in urine is attributed either to increased glucuronidase activity in guinea pig urine or to bacterial deconjugation of urinary metabolites during 24-hour urine collections.