Precocious activation of the pituitary-gonadal axis was induced in immature rats by unilateral electrochemical stimulation of the basal hypothalamus on day 23 of life. In response to this brain stimulus, about 50% of animals ovulated prematurely on either day 27 (96 h after stimulation) or on day 28 (120 h after stimulation), whereas less advanced stages of sexual development (increased ovarian and uterine weights, growth of ovarian follicles) were noted in the others. The combination of the brain stimulus with a single injection of 0.25 mug oestradiol benzoate on day 23 induced ovulation in 15% of rats already on day 26 (72 h after stimulation). When injected alone, this dose of oestradiol benzoate did not induce ovulation, but led to a rapid and sustained increase in the LH/FSH ratio (assessed by radioimmunoassays) in both the anterior pituitary gland and in plasma. Lower priming doses (0.05 mug and 0.125 mug) led to progressively fewer animals ovulating on day 26, but left the number ovulating on day 27 largely unaffected. In contrast, a higher dose (0.5 mug) gave an early response in 15% of the population, but severely reduced the number ovulating in a 10 day follow up period. It is concluded, in agreement with Hohlweg's early concept regarding the feedback effect of oestrogen in immature female rats, that steroid-priming facilitates the ovulatory response to brain stimulation by shifting pituitary gonadotropin output in favour of LH.