Central administration of bombesin has been shown to block the afternoon PRL surge. A circadian change in the activity of tuberoinfundibular dopaminergic (TIDA) neurons that coincides with the onset of the PRL surge has also been found. Whether bombesin acts on the TIDA neuron to affect the PRL surge and where bombesin acts were the focuses of this study. Bombesin (0.75 microgram/3 microliters.rat) was first injected into the lateral cerebroventricle of estrogen-primed ovariectomized rats at 1200 h through preimplanted cannulas. Both the PRL surge and the circadian changes in TIDA neuron activity (using both 3,4-dihydroxyphenylacetic acid concentration and 3,4-dihydroxyphenylalanine accumulation in the median eminence as indexes) were blocked by the treatment. In suprachiasmatic nucleus-lesioned rats, however, both rhythms were absent, and the injection of bombesin exhibited no significant effect. Using microinjection of bombesin (50 ng/0.2 microliter) bilaterally into either the suprachiasmatic or arcuate nuclei at 1200 h, only the former injection effectively blocked both TIDA neuron activity and the PRL surge. We conclude that bombesin may act on the rhythm generation center, the suprachiasmatic nucleus, to exert its effect to disrupt TIDA neuron activity and the PRL surge.