Prolactin (PRL) secretion is under the inhibitory regulation of the tuberoinfundibular dopaminergic (TIDA) system. Short-term elevation in PRL levels has been shown to increase the activity of TIDA neurons, however, the responsiveness of TIDA neurons to chronically elevated serum PRL levels is controversial. The purpose of this study was to investigate the effects of prolonged elevations of serum PRL on TIDA neuronal activity. Female Sprague-Dawley rats (2-3 months old) were ovariectomized and implanted (sc) with haloperidol (HAL), a dopamine receptor antagonist for 6 or 9 months to produce hyperprolactinemia. Ovariectomized, sham-implanted rats were used as controls. Other groups of intact rats were implanted with HAL or sham-implanted for 9 months and then were implanted with PRL-producing MMQ cells for 6 weeks to further increase circulating PRL levels. TIDA neuronal activity was measured in terms of tyrosine hydroxylase (TH) activity in the stalk-median eminence and was correlated with changes in serum PRL levels. After 6 months of treatment, TH activity in HAL-treated rats was 130% higher than that in the control rats. After 9 months of treatment, TH activity in HAL-treated rats was 81% higher than that in control rats. This increase was significantly less than the increase that occurred after 6 months of treatment. Nine months of HAL-induced hyperprolactinemia followed by implantation of PRL-producing MMQ cells, which resulted in very high levels of PRL, did not increase TH activity in the stalk-median eminence. These results demonstrate that hyperprolactinemia over a prolonged period reduces the responsiveness of TIDA neurons, and these effects vary depending on the duration and intensity of hyperprolactinemia.