Melatonin and the ovulatory luteinizing hormone surge.

A newly developed 125I-radioimmunoassay allows for the accurate determination of physiological concentrations of plasma melatonin. Melatonin secretion does not change significantly on the day before and the day of the luteinizing hormone (LH) surge when compared with the early follicular phase. In addition, it was confirmed that the beginning of the LH surge frequently occurs in the morning and is associated with low melatonin values (six of nine women). Supraphysiological melatonin concentrations did not decrease the midcycle LH secretion in four women studied.