The excretion of radioactivity in bile, urine and feces was observed in a female patient treated orally with ethynylestradiol-6,7-3H (EE-3H) and in 1 male and 2 female patients administered ethynylestradiol 6,7-3H-3-cyclopentyl-l′-14C ether (EECPE-3H-14C). During the first 30 hr post administration, urinary and bilary excretion of 3H from EE was 4-5 times that from EECPE, while thereafter the excretion curves were nearly parallel. Over a 6-day period, 59.2% of the 3H dose was excreted by the EE-treated patient, while only about a third as much 3H was excreted by the EECPE-treated patients. A larger percentage of the 14C than of the 3H dose appeared in the urine of all patients dosed with EECPE, while the reverse was seen in the bile. More than 80% of the urinary 14C was non-extractable, whereas biliary 14C and 3H were quantitatively released following β-glucuronidase hydrolysis and solvolysis. Urinary 3H appeared primarily in the glucuronide phenolic fraction following treatment with either EE or EECPE. Biliary radioactivity was usually concentrated in the sulfate fraction, but differed between the 2 treatments. After EE, biliary 3H was primarily phenolic, whereas after EECPE it was mostly in the neutral and acidic fractions in association with 14C. The data suggest that EECPE follows 2 metabolic pathways, the first being cleavage to EE (3H) and 14C-cyclopentanol derivatives which are eliminated in the urine. The second pathway is conjugation while the cyclopentyl ether linkage is still intact and excretion in both bile and urine as doubly labeled compounds. Chromatographic investigation of the urine and bile extracts from the patient given EE showed that unchanged EE was the major excretory product in the glucuronide and sulfate fractions from both fluids. Small amounts of more polar metabolites (one tentatively identified as 6α-hydroxy EE) were also demonstrated. Over the first 25 hr after administration of EECPE, EE was the major urinary metabolite in both glucuronide and sulfate fractions. After 25 hr a 3H-14C containing metabolite (compound III) became quantitatively more important in the urine. The major metabolites in bile from EECPE were EE, compound III and a polar doubly labeled compound, tentatively identified as 6α-hydroxy EECPE. As in urine, the latter 2 metabolites became quantitatively more important with time.