Many studies have established that dopamine (DA) secreted by tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus is the major inhibitory factor controlling prolactin (PRL) secretion from the anterior pituitary. Endogenous opioid peptides (EOPs), mainly the neuropeptide beta-endorphin, facilitate PRL secretion by decreasing TIDA neuronal inhibitory tone in a number of physiological conditions, including pregnancy and lactation. We have previously demonstrated that there are many more c-Fos-expressing neurons than TIDA neurons in the arcuate nucleus, and treatment with naloxone (NAL), an opioid antagonist, activated these neurons in pregnant rats. Our previous data also suggest that the rostral region of the arcuate nucleus is more important than the caudal region in regulating the nocturnal PRL surge in pregnant rats. The aim of this study was to investigate the effects of NAL in regulating TIDA neuronal activity and therefore facilitating PRL secretion during lactation in rats. NAL was continuously infused (0.2 mg/10 microL/min iv) for 1 h before the separated pups returned, and then for 2 or 5 h after the separated pups were returned. Radioimmunoassay (RIA) was used to measure plasma PRL levels, and the immunocytochemical (ICC) staining of c-Fos was performed to detect changes in transcriptional activity of neurons in the hypothalamus. ICC of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis, was performed to visualize TIDA neurons in the arcuate nucleus. The results showed that the peak of the PRL response to suckling was markedly delayed and dampened in NAL-treated rats (p<0.05). The percentage of c-Fos positive TH neurons in the arcute nucleus increased in rats treated with NAL for 5 h after return of pups, but not in rats treated with NAL for 2 h.