The quantitative distribution and localization of bombesin-like immunoreactivity (BLI), by antiserum which cross-reacted fully with gastrin-releasing peptide (GRP), was determined by radioimmunoassay and immunocytochemistry in various regions of female rat urogenital tract. The highest concentration of BLI was found in the vagina, with lower but significant concentrations in the uterus and bladder. Bombesin-like immunoreactive nerve fibres were localized by immunocytochemistry in the smooth muscle layer, around blood vessels and in the submucosa of the vagina and bladder. BLI was separable into two immunoreactive peaks on both gel permeation chromatography and reverse phase HPLC. The major peak emerged exactly in the position of neuromedin C (C-terminal decapeptide of GRP) in both chromatographic systems, while the remaining immunoreactivity emerged close to the position of porcine GRP on gel permeation chromatography, but was easily distinguishable from this on HPLC. This suggests a significant species difference between rat and porcine GRP. Amphibian bombesin and mammalian neuromedin C have previously been shown to have potent biological actions in the rat urogenital tract. The presence of mammalian bombesin-like peptides in nerve fibres of the urogenital tract suggests that they may have a regulatory role in these organs.