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The pattern of hormonal changes just before ovulation was investigated in immature rats induced to ovulate with pregnant mare serum gonadotrophin (PMSG). Oestradiol levels in the blood reached a maximum 42–52 h after PMSG treatment and then decreased dramatically. Pituitary luteinizing hormone (LH) levels remained constant until the time of the critical period (50–52 h) and then fell significantly. Plasma LH levels started to rise at 52 h, reached a peak at 54 h and fell to undetectable levels by 58 h. Plasma progesterone also reached a peak at 54 h and then fell slowly over the following dark period. The timing and pattern of changes in plasma and pituitary hormone levels were very similar to those seen in adult rats before ovulation. There was a positive correlation between the dose of PMSG, the peak concentration of plasma oestradiol and the average number of ova shed.