DNA polymerase activities and DNA content of ovaries from immature intact rats (4-29 days after birth), hypophysectomized rats and hormone-treated hypophysectomized rats were measured. During normal ovarian growth DNA polymerase alpha activity and DNA content of ovaries increased. The polymerase activity decreased gradually after hypophysectomy without any alteration in the DNA content. Administration of ovine FSH (2 micrograms/day) or oestradiol-17 beta (1 mg/day) to hypophysectomized rats enhanced ovarian DNA content and DNA polymerase alpha activity, whereas DNA polymerase beta activity did not change significantly. These results suggest that DNA polymerase alpha participates in DNA synthesis in these ovaries. The specific activity of DNA polymerase alpha (the activity per microgram DNA) in the ovaries increased between 4 and 14 days after birth, and then remained almost constant; the specific activity declined gradually after hypophysectomy. Administration of FSH or oestradiol-17 beta but not of ovine LH, progesterone or testosterone to hypophysectomized rats restored the specific activity. Mixing experiments with different kinds of ovarian extracts suggested that no activators of DNA polymerase alpha were present in the extracts. These results suggest that FSH or oestrogen causes the induction of DNA polymerase alpha accompanied by DNA synthesis during cell proliferation in ovaries of immature rats.