Potential maturational changes in patterns of LH secretion during feed restriction and the LH response to realimentation were examined. Ten sets of four growth-matched littermate gilts were fed ad libitum, to reach either 55, 65, 75 or 85 kg target body weights (one littermate per weight). Gilts were then maintenance fed for 8 days (days 1-8), and to appetite on day 9, and kept in a 9 h light (07:00-16:00 h):15 h dark photoperiod. Blood samples were obtained every 10 min from 07:00 to 16:00 h, and from 16:00 to 24:00 h on day 8 to evaluate any nocturnal rise in LH, and from 07:00 to 16:00 h on day 9 to evaluate the LH response to realimentation. Radioimmunoassays for LH, FSH, melatonin and oestradiol were performed on selected plasma samples. In all three sampling periods, LH and FSH secretion decreased as gilt weight increased, in a quadratic manner. During feed restriction, daytime LH, but not FSH, secretion was lower than at nighttime, irrespective of weight. Daytime LH secretion increased in response to realimentation, irrespective of weight. Plasma oestradiol concentrations were increased by feed restriction and decreased by realimentation but showed little relationship to gilt weight. No consistent relationship was established between plasma melatonin concentrations and ambient illumination. In conclusion, LH secretion exhibits a diurnal rhythm during feed restriction in the prepubertal gilt and, in a similar way to FSH, decreases as puberty is approached. The plane of nutrition influences circulating oestrogen concentrations. The increase of LH secretion following realimentation is not influenced by maturation over the prepubertal period.