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Acute experiments were performed on urethane-anesthetized adult laboratory rats to investigate the effects of microinjections of 10-13-10-4 M bombesin into the solitary tract nucleus on measures of respiration. Bombesin microinjections were found to stimulate respiration, inducing significant increases in the level of pulmonary ventilation, increases in respiratory volume, and increases in the bioelectrical activity of the inspiratory muscles. The most marked respiratory reactions were seen after intermediate peptide doses (10-10-10-7 M). These respiratory effects of bombesin were found to result from its ability to suppress the inspiration-inhibiting Hering-Breuer reflex at the level of the solitary tract nucleus. The fact that ultralow doses of bombesin were active, along with the distribution of endogenous bombesin and its specific receptors in the solitary tract nucleus, and the ability of this peptide to modulate the Hering-Breuer reflex all provide evidence that bombesin is involved in controlling respiration at the level of the dorsal structures of the respiratory center.