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 less than 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.