A RIA for the measurement of plasma equilin (3-hydroxy-1,3,5-(10)7-estratetraen-17-one) and estrone in postmenopausal women and other estrogen-deficient women on exogenous equine estrogen replacement therapy is described. Antiserum against estriol-3,16,17-trihemisuccinate-HSA and high specific activity [2,4-3H]equilin and [6,7-3H]estrone were used in the assay procedure. Specificity of the assay was achieved by separation of equilin from estrone by chromatography on micro-Celite partition columns using silver nitrate as the stationary phase. The sensitivity of the standard curves for both equilin and estrone was 10-20 pg, and the smallest amount of estrone and equilin that could be measured accurately in the plasma was 250 pg/ml for both steroids. The coefficient of variation for both steroids ranged from 1-8%, over a range of 250-5000 pg/ml plasma. The interassay coefficients of variation for equilin and estrone were 4.6% and 2.4%, respectively. After the administration of 10 mg Premarin iv, maximum concentrations of 4 and 11.2 ng/ml for equilin and estrone, respectively, were obtained after 20 min. Thereafter, both steroids disappeared from the plasma gradually. When 10 mg Premarin were administered orally, equilin and estrone appeared in the blood gradually, and maximum levels of 560 and 1400 pg/ml were reached after 3 and 5 h for equilin and estrone, respectively. Equilin gradually disappeared, and by 24 h, only small amounts (125 pg/ml) were detectable. The levels of estrone declined more rapidly, though it was still detectable after 24 h. These preliminary results indicate that equilin sulfate is converted to circulating unconjugated equilin in a manner similar to the conversion of circulating estrone sulfate to estrone.