Recently we reported that the immature female rat spontaneously expressed two daily prolactin (PRL) surges; one surge at 1800 h (diurnal) and the other at 0600 h (nocturnal), but the nocturnal surge ceased occurring as puberty approached. In the present experiment, the effects of ovariectomy and administration of ovarian hormones on the two surges were investigated. Ovariectomy at the age of 23 days did not abolish the two surges occurring on day 29 to 30 and their timing and magnitude were not different from those in the sham-ovariectomized rat. In the intact female rat at the ages of 29-30 days, an injection of 2.5 microgram estradiol benzoate at 1200 h progressively elevated PRL levels, whereas 0.5 mg progesterone injected at 1200 and 2400 h increased the magnitude of diurnal and nocturnal surges. The results indicate that the mechanism for two daily PRL surges in the immature female rat is capable of functioning independently of the ovarian activity, although the latter may exert certain modulatory influence.