In the present study, the possible relationship between melatonin secretion as reflected by nocturnal melatonin excretion (2000 h-0800 h) and the pituitary-adrenocortical axis was investigated. Nocturnal urinary melatonin excretion and plasma cortisol levels were determined in 41 children with weight problems before and after a single oral dose of dexamethasone. A first group of 15 individuals with normal cortisol cycle (12.2 +/- 1.4 at 0800 h and 3.1 +/- 0.5 micrograms/100 ml at 1700 h), and levels below 1.0 microgram/100 ml after dexamethasone, showed a highly significant increase in melatonin excretion during the night following dexamethasone treatment (63.5 +/- 5.5 ng/12 h vs 33.6 +/- 3.0 for the control night, P less than 0.001). This increase was observed from prepuberty to young adulthood (pubertal stages PI-PV). In a second group of 16 subjects with mean cortisol levels similar at 0800 h and 1700 h (10.7 +/- 1.3 and 9.3 +/- 1.5 micrograms/100 ml respectively), but with a normal cortisol suppression after dexamethasone administration, nocturnal melatonin excretion increased from 21.2 +/- 2.1 to 33.4 +/- 3.0 ng/12 h (P less than 0.01). A significant increase was found in prepubertal children (PI) whereas no change was observed at the end of pubertal development (stages PIV-PV). A third group of 10 patients with both low amplitude cortisol cycles (16.2 +/- 2.5 and 10.8 +/- 2.4 micrograms/100 ml) and abnormal cortisol suppression after dexamethasone administration (9.1 +/- 2.4 micrograms/100 ml), showed no increase in melatonin excretion (24.2 +/- 2.7 and 24.9 +/- 3.7 ng/12 h).(ABSTRACT TRUNCATED AT 250 WORDS)