We have studied three, term newborn baboons (Papio papio) to determine whether differential responses to chorionic gonadotrophin (HCG) and ACTH exist with respect to cortisol (F), dehydroepiandrosterone (D), or dehydroepiandrosterone sulphate (DS) secretion as judged by the serum level of these hormones. Serum F, D and DS levels were measured by radioimmunoassay 30, 15 and 0 min prior to constant iv infusion (120 min) of (A) 12 IU ACTH, (B) 2000 USP Units HCG or (C) normal saline.Steroid concentrations measured on samples obtained during the infusion (30, 60 and 120 min) were averaged and compared with the average pre-treatment values. Statistical analyses were by analysis of variance with comparison of individual means by least significant difference procedure. Each animal was examined 6 times (day 1, 2, 3, 7, 8, 9 of life) receiving each treatment twice in a staggered schedule. Mean serum concentrations (\g=m\g/100 ml \m=+-\sd) of D, DS and F prior to treatment were 8.0 \ m=+-\ 3.8; 394 \ m=+-\196; 47.0 \ m=+-\18.7, respectively. ACTH, but not HCG, increased (P < 0.025) serum D, DS and F levels. The response of D and F to ACTH expressed as a per cent of pre-treatment values were similar #$x2266; 100 %) and greater (P < 0.05) than the mean DS percentage (29%) response. These results indicate that ACTH but not HCG is an adrenocorticotrophin in the baboon neonate.