Serum immunoreactive melatonin was measured in thirty-three male subjects with delayed puberty, thirty of whom had simple (idiopathic) delayed puberty (SDP) and three of whom were GH deficient. The results were compared with a control population of twenty-six subjects. Significantly elevated values of serum melatonin were demonstrated in both groups of abnormal subjects as compared with the control population (P less than 0.0001). No significant fall of serum melatonin was noted in the subjects with simple pubertal delay prior to the onset of puberty. However, there was a significant fall of serum melatonin in these subjects between mid and late puberty (P less than 0.025). The results suggest that whilst melatonin may inhibit puberty, a fall in serum levels is not essential for the commencement of puberty in man.