We evaluated the 24-h profile of serum melatonin (MT) in 16 patients with Cushing's syndrome (11 pituitary- and 5 adrenal-dependent) aged 38.1 +/- 13.8 years and in 19 patients with acromegaly due to GH-secreting pituitary adenoma aged 52 +/- 11.9 years. Thirty-three healthy subjects aged 30 +/- 7 years and 26 healthy subjects aged 59 +/- 12 years served as age-matched controls for the two groups of patients, respectively. All the patients had active disease and were off treatment; none had overt hypopituitarism. Blood drawings were collected at 08:00, 12:00, 16:00, 20:00, 22:00, 24:00, 02:00, 04:00 and 06:00 h in controls and patients. Serum MT, cortisol, GH and IGF-I were measured by specific RIA kits and ACTH by IRMA. Cosinor rhythmometry was employed for chronobiological analysis. The acromegalic patients displayed, as a whole group, higher 24-h mean MT levels with respect to age-matched controls (M +/- SD: 52.9 +/- 14.6 vs 32.9 +/- 8.0 pg/ml, p < 0.01) with normally synchronized MT profiles in 11/19 patients. No correlation was apparent between MT and either GH (24-h mean) or IGF-I. In the patients with Cushing's syndrome the circadian profile of serum MT was maintained in 10/16 cases; serum MT levels were comparable to those of controls (48.8 +/- 14.0 vs 50.0 +/- 16.0 pg/ml, NS) without any difference between pituitary- and adrenal-dependent forms. No correlation was found between MT and either ACTH or cortisol levels.(ABSTRACT TRUNCATED AT 250 WORDS)