In other energy-restricted mammals, a single large meal restores luteinizing hormone (LH) pulsatility within a few hours. To determine whether this is so in women, we measured LH pulsatility during the 5th day of low energy availability [dietary energy intake - exercise energy expenditure = 10 kcal . kg lean body mass (LBM)-1 . day-1] and during a 6th day of aggressive refeeding (90 kcal . kg LBM-1 . day-1) in 15 meals providing 4,100 kcal for an energy availability of 75 kcal . kg LBM-1 . day-1. Low energy availability raised beta-hydroxybutyrate 1,000% (P < 0.001) and reduced plasma glucose 15% (P < 0.01), insulin 63% (P < 0.001), and triiodothyronine 22% (P < 0.005). In five of eight subjects, low energy availability also unambiguously suppressed LH pulse frequency 57% to 8.2 +/- 1.5 pulses/24 h (P < 10(-4)) and raised LH pulse amplitude 94% to 3.1 +/- 0.3 IU/l (P < 10(-4)), levels below the 5th and above the 95th percentile, respectively, in energy-balanced women. Aggressive refeeding restored beta-hydroxybutyrate, glucose, and insulin, but not triiodothyronine. In the five women with unambiguously disrupted LH pulsatility, aggressive refeeding had no effect on LH pulse amplitude (P > 0.9) and raised LH pulse frequency only slightly (2.4 +/- 0.6 pulses/24 h, P = 0.04) and not above the fifth percentile. This striking contrast between women and other mammals may be another clue to the unidentified mechanism mediating the effect of energy availability on LH pulsatility.