Diurnal variations in serum unconjugated estradiol-17β (SE2), serum total estriol (SE3), total urinary estrogens (UE), and UE/creatinine ratios (E/C) were determined on samples obtained at 4-hr intervals from 22 women in the third trimester of normal pregnancy. Analysis of variance indicated significant variations in SE2 (p < 0.001), UE (p < 0.01), and E/C (p < 0.05). Multiple comparison of means showed maximum values at 12 pm to be significantly greater than minimum values at 12 am (p < 0.01). The SE3 rhythm was less marked with individual daytime (8 am–12 pm–4 pm) means significantly greater (p < 0.001) than those at nighttime (8 pm–12 am–4 am). It is proposed that this rhythm reflects changes in availability of estrogen precursors known to be secreted rhythmically by the maternal adrenal.