The purpose of the present investigation was to correlate changes in ovarian catecholamines and serum pituitary hormones and sex steroids. As the experimental model we used the prepubertal rat injected with 7.5 IU PMS on day 28 at 0900 h. Within 24 h after the injection, ovarian weight increased, and on day 30 at 2400 h, it was twice that of controls. On day 31, all of the PMS-treated rats but none of the controls had ovulated, and the average number of ova was 9. Serum estradiol in these rats was elevated significantly on the afternoon of day 30 and was followed by a preovulatory surge of LH and FSH. Serum progesterone was also increased on the evening of day 30, and an elevation in serum PRL on the morning of day 30 in the PMS-treated rats was evident, whereas serum GH was unchanged. Ovarian norepinephrine (NE) dropped from 65.5 +/- 6.9 pg/mg ovary (mean +/- SE) before the PMS injection of 14.4 +/- 1.5 pg/mg ovary on the evening of day 30. This depletion proceeded in two apparent phases: an initial drop within 12 h after PMS injection, and a further reduction coincident with the gonadotropin surge. Throughout this period, ovarian NE in the controls increased 50--60%. There was a 2-fold reduction in ovarian dopamine in the PMS-primed rats, but the levels were 10-fold lower than those of NE. The data document that the hormonal profiles and the number of ovulating follicles of the prepubertal rat primed with a low dose of PMS resemble those of the normal cycling rat. A significant depletion of ovarian catecholamines was observed during the preovulatory period, which is temporally related to increases in serum gonadotropins.