Pituitary adenylate cyclase activating peptide is a new member of the vasoactive intestinal polypeptide family of peptides which is present in the brain as well as neuronal elements of a number of peripheral organs. Pituitary adenylate cyclase activating peptide occurs in two forms, pituitary adenylate cyclase activating peptide-38 and the C-terminally truncated 27 amino acid form, pituitary adenylate cyclase activating peptide-27, both derived from the same precursor which in addition gives rise to a structurally-related peptide, pituitary adenylate cyclase activating peptide-related peptide. Using specific radioimmunoassays for pituitary adenylate cyclase activating peptide-38, pituitary adenylate cyclase activating peptide-27 and pituitary adenylate cyclase activating peptide-related peptide we found that all three pituitary adenylate cyclase activating peptide-precursor-derived peptides were present in tissue extracts from the ureter, the urinary bladder and the urethra. Pituitary adenylate cyclase activating peptide-38 was the dominating peptide with the highest concentration in the ureter. When extracts from the urinary bladder were fractionated by reverse phase high pressure liquid chromatography immunoreactive components corresponding to synthetic pituitary adenylate cyclase activating peptide-38, pituitary adenylate cyclase activating peptide-27 and pituitary adenylate cyclase activating peptide-related peptide were identified with the respective antisera. By immunohistochemistry, using a specific monoclonal mouse anti-pituitary adenylate cyclase activating peptide antibody, pituitary adenylate cyclase activating peptide-immunoreactivity was shown to have a widespread distribution in the rat urinary tract, localized exclusively to nerve fibres. No immunoreactive neuronal cell bodies were observed in any of the tissues. Pituitary adenylate cyclase activating peptide was shown to be located in varicose nerve fibres associated with blood vessels and smooth muscle. The majority of pituitary adenylate cyclase activating peptide-positive nerve fibres and bundles were, however, present in subepithelial plexuses from which delicate varicose nerve fibres entered the urothelium. Double immunostaining for pituitary adenylate cyclase activating peptide and a marker for sensory neurons, calcitonin-gene related peptide, disclosed that the two peptides were almost completely co-localized while the co-existence between pituitary adenylate cyclase activating peptide and the structurally related peptide vasoactive intestinal polypeptide, was scarce. Neonatal capsaicin-treatment caused a marked reduction in the concentration of immunoreactive pituitary adenylate cyclase activating peptide in all regions of the rat urinary tract, being most prominent in the ureter. By immunohistochemistry it was shown that the sensory neurotoxin caused a reduction in the number and intensity of pituitary adenylate cyclase activating peptide-immunoreactive nerve fibres in all organs of the urinary tract which was most prominent in the epithelial and subepithelial layers. Identical changes were observed for the calcitonin-gene related peptide-containing nerve fibres, while vasoactive intestinal polypeptide-positive nerve fibres were unaffected by capsaicin-treatment. In conclusion pituitary adenylate cyclase activating peptide is present in the rat urinary tract mainly in the form of pituitary adenylate cyclase activating peptide-38. Immunoreactive nerve fibres were associated with the epithelium, blood vessels and smooth musculature. Pituitary adenylate cyclase activating peptide was almost completely co-localized with calcitonin-gene related peptide and by neonatal capsaicin treatment the two peptides were identically affected. The findings suggest that pituitary adenylate cyclase activating peptide is a sensory neurotransmitter in the rat urinary tract.