The effect of oestradiol-17beta (administered in Silastic capsules) on gonadotrophin secretion in long-term gonadectomized rats has been investigated. In female rats, a daily afternoon surge of LH occurred which could be blocked by administering sodium pentobarbitone at 13.00 h. This, together with the fact that there was no significant difference between the LH-response to LH-releasing factor in the morning compared with the afternoon, supports the view that the LH surges are due to a neural rhythm. There was no significant diurnal variation in plasma FSH concentration, but an apparent diurnal variation was disclosed by administering sodium pentobarbitone on alternate days. There was no diurnal variation in plasma gonadotrophin concentrations in male animals implanted with an oestradiol capsule, or in animals of either sex bearing empty capsules.