Hypophysectomy performed in 25- to 26-day-old Wistar rats leads within 1 year to the formation of ovarian testis-like tubes and epithelial cellular cords which are typical structures in the senile ovaries of normal 24-month-old rats. Testis-like tubes represent an unusual late stage of follicular degeneration; the origin of the cords is more complex; the revival of cell differentiation from stroma and/or rete ovarii in the absence of the pituitary is hypothesized. At 16 months of age, cords proliferate and are responsible for ovarian weight increase to almost twice the minimal weight seen 4 months after the operation. Variability in cord proliferation is considerable from one rat to another, but also between the two ovaries of the same animal. Thus, an intrinsic age-related ovarian factor is implicated in cord proliferation. From this study, it is inferred that during intact rat senescence, intrinsic ovarian aging is responsible for the proliferation of cords, whereas their induction depends on a hypothalamic-hypophyseal imbalance occurring at about 1 year of age.