We have evaluated the relationship between beta-endorphin (beta-EP) and melatonin (MEL) secretion as determined at 3 h intervals over a 24 h period in 27 normotensive obese women of different phenotypes (12 showing gynoid-type and 15 showing android-type of adipose tissue distribution) as well as in 12 healthy volunteers with normal body weight, aged 30 to 40 years. A considerable increase of mean 24 h beta-EP secretion (mainly due to elevated afternoon and evening levels) and total absence of beta-EP circadian rhythm were observed in all obese patients. Mean 24 h MEL concentrations were markedly higher (mainly due to increased daytime levels) in all obese patients while the disturbances of MEL secretion in the form of acrophase shift and (or) suppression of its rhythmicity were observed especially in obese women with android phenotype. Circadian beta-EP levels correlated positively with BMI and WHR values and negatively with circadian MEL concentrations. In contrast, no significant correlation was found between the values of BMI, WHR and MEL levels. Our data indicate that alterations of beta-EP secretion occurring in obese women could play a role in inducing disturbances of MEL secretory pattern.