In order to clarify the role of GH on ovarian function, recombinant human GH (rhGH) was administered to adult spontaneous dwarf rats (SDR) for 14 days. The SDR were sacrificed on the day of estrus and the effects of rhGH on ovarian weight, follicular population and the viability estimated by 5-bromodeoxyuridine (BrdU) incorporation, were studied. RhGH administration caused a significant gain in body weight of the SDR, and the ovarian weight of the GH-injected SDR showed a significant increase in comparison with saline injected SDR. The total population of follicles was significantly greater in the GH-treated group than in the control, and the difference was attributed to the increase in the number of large follicles more than 500μm. Total uptake of BrdU into granulosa cells was significantly higher in GH-treated rats than in the control, and, in particular, the intermediate-sized follicles (300-500μm) of the GH-treated rats showed signs of high incorporation of BrdU. These results suggest that hGH acts on intermediate follicles, stimulates their viability and increases the number of growing follicles.