The pituitary-adrenal axis activity was evaluated in 43 patients, treated for congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency, by measuring plasma ACTH, 17-hydroxyprogesterone (17-OHP), testosterone, and aldosterone. Dynamic studies were performed by injecting 250 micrograms synthetic ACTH im and collecting blood samples 1 h later for steroid analysis. Twelve to fourteen hours after the last hydrocortisone dose given the evening before, plasma ACTH fluctuated widely from less than 10-475 pg/ml, 17-OHP exceeded normal values and varied from 1-275 ng/ml, while testosterone ranged from 3-151 ng/100 ml. The correlations between ACTH and 17-OHP (n equal 61, r equal 0.665, P less than 0.001) and between 17-OHP and testosterone (n = 43, r = 0.761, P less than 0.001) were good, while that between 17-OHP and aldosterone (n = 64, r = 0.512, P less than 0.001) was rather poor. One hour after ACTH injection, the mean level of 17-OHP was significantly increased as compared to the mean basal level [96.8 ng/ml +/- 10.6 (SE) as compared to 67.0 ng/ml +/- 8.1 (SE)]. However, only 12 out of the 48 tests showed a positive response equal to or greater than 100%, and the majority of these responses (10 out of 12) occurred when basal levels of 17-OHP were between 10-70 ng/ml. This suggests that when basal levels fall outside these values, the pituitary-adrenal axis is either too inhibited or too stimulated to react to exogenous ACTH. Of the 48 tests where 17-OHP was measured, 23 had basal level values within these limits, the mean being 40.3 ng/ml. The corresponding mean ACTH level was 99 pg/ml with a wide range (1-230 pg/ml). On the other hand, in prepubertal children who exhibited 17-OHP concentrations between 10-70 ng/ml, testosterone varied from 3-30 ng/100 ml, with a mean of 16.0 ng/100 ml +/- 1.9 (SE) which is not different from the mean level found in normal children [14.0 ng/100 ml +/- 1.3 (SE)]. Thus, under the influence of endogenous ACTH which is moderately increased, 17-OHP concentrations far exceed normal values, whereas plasma testosterone seems to be unaffected.