We previously reported that the posterior pituitary dopaminergic system participates in the inhibition of prolactin (PRL) secretion in both male and lactating female rats. However, posterior pituitary lobectomy (Lobex) of urethane-anesthetized cycling rats resulted in an elevation in plasma PRL for a short time only. This raises a question regarding the importance of input from the posterior pituitary to the control of PRL secretion during the estrous cycle. The objectives of this study were (1) to examine the chronic effects of Lobex on plasma PRL levels in conscious rats and (2) to determine whether the absence of input from the posterior pituitary interferes with estrous cyclicity. Lobex or sham lobectomy were performed under Brevital anesthesia in estrous rats. Blood was collected from a jugular cannula at hourly intervals on the day of surgery and at 09.00, 13.00, and 17.00 h during the following 4 days. Daily water consumption and vaginal cyclicity were monitored for 14 and 20 days, respectively. Within 2 h after Lobex, the plasma PRL levels rose 3- to 4-fold and remained elevated for 3 days before declining to near control levels on the 4th day. None of the Lobex rats resumed cyclicity within 3–4 days, 50% had an interruption of cyclicity for 4–10 days, and the remainder were noncyclic for more than 11 days. Upon resumption of cyclicity, Lobex rats had 11.3 ± 0.4 oviductal ova which is within the normal range for intact ovulating rats. In contrast, 60% of the sham-operated rats resumed cyclicity within 4 days with an afternoon proestrous surge of PRL, whereas 30% became pseudopregnant, as judged by diurnal peaks of plasma PRL levels and a sustained leukocyte smear for 11–13 days. The daily water consumption of Lobex rats was 3-fold higher than that of sham-lobectomized rats for at least 2 weeks. It is concluded that: (1) Lobex results in a prolonged elevation of plasma PRL levels in conscious rats; (2) the eventual decline in plasma PRL is more likely due to a compensatory activation of the hypothalamic dopaminergic system by the Lobex-induced rise in plasma PRL than to a regeneration of the posterior lobe or restoration of its function; (3) the Lobex-induced PRL rise appears to be the cause of interruption of cyclicity, and (4) the data are consistent with the hypothesis that the dopaminergic inhibition of PRL secretion involves two interdependent systems: one consists of the hypothalamus – long portal vessels – and the second comprises of the posterior pituitary -short portal vessels. Depending upon the endocrine conditions and the nature of hormonal or neuronal stimuli, either one or both systems may be activated.