Induction of estrous cycles in old non-cyclic rats by progesterone, ACTH, ether stress or L-dopa

Cycling was induced in old non-cyclic, constant estrous rats by daily injections of progesterone, ACTH or L-dopa or by subjection to ether stress. Progesterone and ACTH were the most effective agents used for re-establishing estrous cycles in these rats. Most of the progesterone- and ACTH-treated rats showed regular cycles, and their ovaries had many corpora lutea; they also showed proestrous serum LH surges. Ether stress and L-dopa mostly induced irregular cycles and fewer corpora lutea in the ovaries; a smaller number of these rats showed proestrous LH surges. After treatment with each of these agents was discontinued, most of the rats returned to constant estrus or irregular cycling.