An experiment was conducted to define the seasonal changes in LH secretion in ovariectomized does with oestrogen implants and the effect of immunization against melatonin. Fifteen mature Australian cashmere goats were ovariectomized and given either no further treatment or one or two implants containing oestradiol; another similar group of 15 does were immunized against melatonin before ovariectomy and oestrogen treatment. LH concentrations and livemasses were recorded every week for 2 years. Livemasses of both groups showed a distinct seasonal pattern with a summer maximum and a winter minimum irrespective of treatment. LH concentrations also showed distinct seasonal patterns with a significant interaction between the number of implants and the time of the year. In the nonimmunized does, the presence of a constant low dose of oestrogen (one implant) resulted in low concentrations of LH except from May to August, the normal period of spontaneous ovulatory activity in intact does. In contrast, nonimmunized does receiving a high dose of oestrogen (two implants) showed a rise in LH concentrations in February, and concentrations remained high until August. Immunization against melatonin abolished this differential LH secretory pattern, and both doses of oestrogen were associated with a short period of high LH concentration between May and September. These results indicate that a negative feedback effect of oestrogen results in low LH secretion for most of the year and that hypothalamic sensitivity to LH decreases for only a short period between May and August. It is suggested that the early increase in LH secretion in does with two oestrogen implants is due to a positive oestrogen feedback mechanism that may depend on a photoperiodic signal for activation.