The aim of this study was to determine the normal relationship between ACTH and cortisol secretion in children. Fourteen children (nine male, five female; median age 11·3 years) were hospitalized and blood samples were taken every 20 min for 24 h. A circadian rhythm was observed with median 0900 h and midnight ACTH values of 1·80 and > 0·97 pmol/l, and for cortisol 296 and 62 nmol/l respectively. The median (range) areas under the curve for ACTH and cortisol were 29·7 (9·0–53·8) pmol/l/h and 5114 (3562–8630) nmol/l/h respectively. There were no significant differences detected for ACTH and cortisol secretion between males and females, or between prepubertal (n= 9) and pubertal subjects (n= 5). Using a novel form of time series analysis we have shown that both ACTH and cortisol are secreted with a dominant periodicity of 0·7–1·0 h, representing 24–34 secretory episodes of ACTH and cortisol In 24 h. For cortisol, but not ACTH, there is a significant secondary periodicity of 2–3·2 h. To look for shared periodicities we have used the technique of coherency. This reveals that for six of the children ACTH and cortisol are secreted with a significant shared periodicity of 0·8–1·0 h, and for a further five children a similar secondary shared periodicity Is present. Therefore in normal children ACTH and cortisol secretion are Interdependent and episodic but are not influenced by either pubertal status or gender.