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Experiments were performed to determine whether the initial rise in the level of LH in peripheral plasma which occurs during its preovulatory surge in plasma may modulate the subsequent patterns of rising of LH and FSH which occur during pro-oestrus. Two cannulae were inserted into the right atrium of the heart of pro-oestrous rats previously given phenobarbitone to block the preovulatory surges of LH and FSH in plasma. One cannula was used for blood collection and injections and the other for infusions. Vehicle, 0·2 or 2·0 µg rat LH, or 1·0 µg rat FSH was injected 60 min before a 90 min infusion of 0·9% saline or of LH releasing hormone (LH-RH) which was infused at a rate of 50 ng/h to simulate the LH and FSH surges. Plasma LH rose more than 12-fold or 125-fold 5 min after injection of 0·2 or 2·0 µg rat LH respectively. At 60 min, the level of LH in plasma was basal or raised about fourfold respectively. The plasma LH and FSH responses to LH-RH in these rats were similar to those of control animals. Infusion of LH-RH caused levels of LH in plasma to increase about fourfold during an initial rising phase. This was followed by a marked increase in levels of plasma LH during a rapid rising phase, an effect attributed to self-priming by LH-RH. Plasma FSH was not raised after 25 min but was increased 60 min after the start of infusion of LH-RH. Injection of FSH did not alter the plasma LH response to LH-RH. The results suggest that the delayed rise in plasma FSH in response to LH-RH is not influenced by, nor is it the result of, a prior increase in levels of LH in peripheral plasma and that the marked increase in plasma LH attributed to self-priming by LH-RH is not the result of a positive feedback of peripheral plasma LH on its own secretion.