Hypogonadotropic hypogonadism (IGD) and constitutional delayed puberty (DP) share a common pathophysiologic process, i.e. GnRH deficiency. Both conditions are heterogenous and exhibit different grades of GnRH deficiency. To discern whether these disorders of GnRH deficiency are associated with altered melatonin secretion profiles, we compared untreated young males IGD (n = 7) and DP (n = 7) to normal pubertal male controls (n = 6). Serum samples for melatonin, LH, and prolactin concentrations were obtained every 15 min from 1900 h to 0700 h in a controlled light-dark environment with simultaneous sleep recordings. Mean (+/- SD) darktime nocturnal melatonin levels were significantly higher in IGD (259 +/- 73 pmol/L) and DP (217 +/- 29 pmol/L) compared with 182 +/- 69 pmol/L in controls (P < 0.02). So were the mean (+/- SD) peak melatonin levels (410 +/- 117, 327 +/- 97 and 298 +/- 95 pmol/L in IGD, DP, and controls, respectively (P < 0.05). Integrated nocturnal melatonin secretion values (AUC) were also higher in IGD and DP (168 +/- 45 and 134 +/- 28) compared with 119 +/- 45 pmol/min.1 x 10(3) in controls (P < 0.02). The time of melatonin peak and the time of onset of the nocturnal melatonin rise were observed earlier in IGD and DP. Light-time mean (+/- SD) serum melatonin levels were similar in all three groups. No correlations were found between melatonin and LH levels, nor between melatonin and prolactin levels. These data indicate that melatonin secretion is increased in male patients with GnRH deficiency. The lack of correlations between melatonin and LH suggest that circulating sex steroids, rather than LH, modulate melatonin secretion in a reverse fashion.