The effect of ACTH and metyrapone administration on excretion of estriol, 17- hydroxycorticosteroid (17-OHCS), 17-ketosteroid (17-KS), pregnanediol (P-diol) and pregnanetriol (P-triol) was determined in 16 women in the last trimester of pregnancy. The response to ACTH and metyrapone, measured as 17- OHCS excretion, was diminished compared to that reported for nonpregnant women, but was approximately the same when either agent was given. This suggests that altered ACTH release is not the cause of this diminished response. Estriol excretion decreased significantly (p <.00l) following ACTH and increased (p<.1) in 10 of 16 following metyrapone. 17-KS and P-triol changes corresponded to those found for estriol rather than to those found for 17-OHCS. This finding suggests that cortisol from the maternal adrenal may inhibit the fetal pituitaryadrenal axis during pregnancy.