The circadian rhythm of plasma proopiocortin-related peptides was studied in 15 heroin addicts and in 6 sex- and age-matched controls. ACTH, β-lipotrophin, (β-LPH), β-endorphin (β-EP) and cortisol were measured by RIA either directly (cortisol), or after plasma extraction (ACTH) and Sephadex G-75 gel chromatography (β-LPH and β-EP) every 4 h from 8 a.m. to 8 p.m. and again at 8 a. m. the next morning. The means of the two 8 a.m. measurements of β-LPH (2.67 ± 0.34 fmol/ml, mean ± se), ACTH (2.74 ± 0.71) and cortisol (218 ± 31 pmol/ml) levels in heroin addicts were significantly lower than those in controls (6.28 ± 0.61, 10.1 ± 0.74 and 364 ± 27, respectively, P < 0.01) while β-EP concentrations in heroin addicts (5.1 ± 0.6) were similar to those of healthy volunteers (6.44 ± 0.56). In controls, all three peptides and cortisol show a circadian rhythm of secretion, the lowest values being in the evening and the highest ones in the morning. Heroin addicts partially lack this phenomenon showing constant levels of the three proopiocortin-related peptides throughout the day, with a slight but significant decrease of plasma cortisol. In the 7 subjects who took heroin throughout the study, no systematic changes were observed in the three proopiocortin-related peptides, while it seems that this group of addicts shows a cortisol decrease in the evening to a lesser extent than subjects receiving methadone maintenance only. These data show a decreased basal β-LPH, ACTH and cortisol secretion in heroin addicts, despite normal β-EP levels. This would indicate that anterior pituitary proopiocortin synthesis and/or secretion is affected in these subjects, and that there is also some impairment in the structures controlling the circadian rhythmicity of ACTH and related peptides.