The effects of hypophysial stalk transection and mechanical stimulation of the hypophalamus on luteotropin (LtH) secretion were compared in 75 female rats. Stalk section, accompanied by an effective barrier to vascular regeneration, brought about continuous secretion of LtH, expressed by a persistent pseudopregnant state (10 rats). Representative animals exhibited a decidual reaction to uterine trauma as late as 74 days postoperatively. When the stalk transection was ineffective, due to faulty placement of the barrier, only a temporary pseudopregnancy ensued and vaginal cycles returned after 8-37 days (15 rats). Simple mechanical stimulation of the median eminence region during vaginal estrus was followed in all cases (11 rats) by temporary pseudopregnancy. On the other hand, cycles were uninterrupted in 10 to 12 animals by comparable manipulation of the temporal lobe. When performed during diestrous day 1, mechancial stimulation of the hypothalamus also resulted in pseudopregnancy (in 5 of 6 animals). During diestrous day 2 this procedure did not alter vaginal cycles (in 6 of 8 animals). The remaining 2 rats underwent an additional estrous cycle following the hypothalamic stimulation and then exhibited a pseudopregnancy. The results obtained with a group of animals similarly " stimulated" during proestrus were inconsistent. Thus, temporary pseudopregnancy can be induced by simple stalk transection or even mechanical contact with the basal hypothalamus. In order for a persistent LtH secretion to occur, the pituitary gland has to be effectively separated from the hypothalamus by a barrier to vascular regeneration.