Plasma melatonin levels were determined by a sensitive RIA at 30 min intervals for 24h in 7 women with functional hypothalamic amenorrhea (HA) and in 7 age and season matched normal cycling women in the early follicular phase (NC). While daytime melatonin concentrations were nondetectable in both groups, the integrated nocturnal levels were 3-fold greater in HA (244 +/- 58 (SE) vs 74 +/- 32 pmol-min/Lx10(3), p less than 0.005). This melatonin increase in HA was due to an elevated peak amplitude (p less than 0.01) and extended duration (p less than 0.05). The latter was mostly due to a significant delay in the offset time of the amplified nocturnal melatonin secretion.