Composite microassays for plasma progesterone (P), 17alpha-OH-progesterone (17P), estrone (E1), 17beta-estradiol (E2), and estriol (E3) were developed. Steroids were extracted from plasma samples with diethyl ether, and were separated from each other through two steps of sephadex LH-20 microcolumn chromatography (benzene:methanol 85:15, and n-hexane: benzene: methanol 80:10:10), prior to assays by radioimmunoassay (P, E1, E1, and E3) and competitive protein binding assay (17P). Steroids were recovered satisfactorily through these procedures (mean recovery; P:107.6%, 17P:102.3%, E1:88.2%, E2:84.1%. E3:78.5%). The detectable range for P, 17P, E1, E2, and E3 were 0.01-2 ng/tube, 0.1-10 ng/tube, 10-2000pg/tube, 10-2000 pg/tube, and 0.5-100 ng/tube. The interassay coefficient of variations were less than 10.7%, 15.2%, 17.8%, 11.4%, and 28.1%, respectively. These steroids were measured daily in 4 menstrual cycles from 4 normally menstruating women. Plasma FSH and LH were quantitated previously. The following results were obtained; 1) Plasma P elevated from around LH surge (Day O), and reached a peak on day +5 approximately +7 with the values of 12.74 +/- 2.34 ng/ml (Mean +/- S.D.). 2) Slight decrease in P levels was noted on day +8 approximately +9. 3) A peak in 17P was observed in the preovulatory phase (day -3 approximately -1) with the values of 0.6 approximately 1.3 ng/ml in three of four cases. 4) Changes of 17P during the luteal phase were paralleled to those of P with a peak of 1.16 +/- 0.31 ng/ml on day +5 approximately +7. 5) No remarkable patterns were found in E1 levels throughout the menstrual cycle. 6) A sharp peak in E2 was detected in the preovulatory phase (day -1 approximately 0) with the values of 709.2 +/- 95.9 pg/ml. 7) The second peak of E2 with 378.6 +/- 140.7 pg/ml was observed in the late luteal phase (day +8 approximately +12). 8)E3 was not detected in all samples. The interrelationship between steroids and the correlation with the morphological changes of the ovaries in the normal menstrual cycle are discussed. In the follicular phase, the theca interna cells around the maturing follicle may be growing under the influence of pitiutary gonadotropins to secrete large amounts of 17P and E2, which may possibly affect the pituitary for LH surge, followed by ovulation. In the luteal phase, both the granulosa cells and theca intera cells are luteinized, which may produce and secrete large amount of P, 17P, and E2.