The aim of the study was to evaluate the interrelation between the levels of growth hormone (GH), prolactin (PRL), somatomedin-C (Sm-C) and melatonin determined at 3 h intervals over a 24 h period (8.00, 11.00, 14.00, 17.00, 20.00, 23.00, 2.00 and 5.00) in 8 acromegalic patients after radiotherapy. The circadian rhythms and the interrelation between hormone and Sm-C levels were evaluated using Halberg's cosinor method and the correlation analysis, respectively. We observed a total abolition of melatonin rhythm with significantly lower nocturnal values in all patients which could be due to earlier radiotherapy.