Ionophoresis of the smaller bombesin-like peptides (gastrin releasing peptide [GRP]-(18-27), neuromedin B, and bombesin) evoked responses from 30-60% of hamster suprachiasmatic nucleus cells recorded in a hypothalamic slice preparation, depending on the circadian phase. We also demonstrated for the first time that the putative bombesin-like peptide receptor antagonists [D-F5,D-Phe6,D-Ala11]bombesin-(6-13)methyl ester (BIM 26226) and [D-Phe6,Des-Met14]bombesin-(6-14)ethyl amide can be applied ionophoretically to block physiological responses to bombesin-like peptides. Together with earlier findings, these results show that bombesin-like peptides administered by several methods can potently alter the firing rates of hamster suprachiasmatic nucleus neurons in vitro. These results indicate that bombesin-like peptides affect suprachiasmatic nucleus cells and could play a role in modulating suprachiasmatic nucleus-mediated circadian rhythm entrainment.