To investigate whether the function of pinealocytes is altered in obesity, nocturnal melatonin (MT) secretion was determined in nine healthy subjects and compared with that of eight obese individuals. Serum MT levels were measured every second hour between 6:00 PM and 8:00 AM, and total nocturnal MT secretion (as reflected by the MT incremental area), MT peak time, and nocturnal urinary MT excretion were determined. None of these parameters differed significantly in the two groups. The obese subjects were reinvestigated after 2 days of complete fasting. This caused a decrease in body weight and basal blood glucose levels of 2.6 +/- 0.2 kg (mean +/- SEM, P less than .001) and 1.5 +/- 0.2 mmol/L (P less than .001), respectively, whereas serum cortisol levels remained unchanged. Short-term fasting reduced nocturnal MT secretion, as evidenced by MT incremental areas, which were reduced from 2.01 +/- 0.26 before fasting to 1.64 +/- 0.26 nmol/L.h after fasting (P less than .02). MT secretion peaks were reached simultaneously, and urinary MT excretion values did not change significantly in fasting. To see whether glucose supplementation during short-term fasting would normalize nocturnal MT secretion, we gave an additional seven obese subjects eight small oral doses of glucose (each dose, 0.5 g/kg body weight) at regular intervals during a 2-day fast. Their body weight decreased by 2.8 +/- 0.4 kg (P less than .001), but blood glucose and cortisol concentrations were similar before and after the glucose-supplemented fast, as was the nocturnal MT secretion.(ABSTRACT TRUNCATED AT 250 WORDS)