Daily injections of epinephrine, 1-dopa, or iproniazid for 25 days into 20- to 23-month-old constant estrous Sprague-Dawley rats initiated regular estrous cycles in most rats. On termination of treatment, a few rats continued to cycle but the majority returned to a state of constant estrus or went into a prolonged diestrus. It is concluded that all three drugs restored regular cycling in old constant-estrous rats by correcting a catecholamine deficiency in the hypothalamus, thereby increasing release of LH and FSH and decreasing release of prolactin.