Reproductive capacity of aging female rats.

Multiparous female Long-Evans rats, 20 months or older, were seen to progress from irregular estrous cycles to a constant-estrous syndrome to a series of irregular pseudopregnancies to an anestrous state. An animal in constant estrus was characterized by ovaries with well developed and sometimes cystic follicles, no corpora lutea, an estrogen-stimulated uterus, and an anterior pituitary that appeared normal. Repeatedly pseudopregnant rats had long diestrous periods of variable length, ovaries with many corpora lutea, uteri with numerous secretory glands, and anterior pituitaries that often showed hemorrhagic or small tumorous areas. Anestrous rats had small, atrophic ovaries with no obvious follicular or luteal elements, atrophic uteri, and large pituitary tumors. Twice-daily injections of L-dopa induced the resumption of regular or irregular estrous cycles in most constant-estrous but not in pseudopregnant or anestrous rats.