The radioreceptor assay was used to examine the distribution of bombesin receptors in the rat brain. The highest concentrations of receptors appeared to be associated with limbic forebrain and midbrain structures such as the hippocampus, amygdala, hypothalamus and the periaqueductal gray matter. The caudate-putamen of the extrapyramidal motor system and the forebrain also exhibited high bombesin binding. Intraventricular injections of bombesin (0.1, 1.0 and 10 microgram) produced a dose-dependent increase in locomotor activity in rats. Injections of bombesin into the periaqueductal gray matter produced an antinociceptive reaction in the hot-plate as well as the tail-flick test. This apparent analgesia was not antagonized by naloxone.