Pituitary adenylate cyclase activating polypeptide (PACAP) is a newly discovered regulatory peptide related to vasoactive intestinal peptide and is found widely distributed throughout peripheral tissues and the central nervous system. We examined the ability of its two major forms of 38 (P38) and 27 (P27) amino acid residues to cross the murine blood-brain barrier. After i.v. injection, [125I]P27 had a unidirectional influx constant (Ki) into the brain of 2.13 (10(-3)) ml/g/min with no saturable component to entry. Despite being larger and less lipophilic, [125I]P38 entered the brain more than 30% faster by a saturable transport system. A peptide-T analog related to PACAP that has its own saturable transport system did not inhibit the entry of [125I]P38, but did alter its binding to vascular receptors. Despite the greater Ki of [125I]P38, a larger percentage of the i.v. dose of [125I]P27 entered the brain due to favorable pharmacokinetics. However, [125I]P38 was more resistant to degradation within the brain and, after correction for degradation, its Ki increased to 16.5 (10(-3)) ml/g/min. The influences of peripheral degradation and sequestration by capillaries were negated by use of the brain perfusion and capillary depletion methods. These showed that the Ki into the brain interstitial fluid/parenchymal space for [125I]P38 was 15.3 (10(-3)) ml/g/min and was again inhibited with unlabeled P38. Both PACAPs were transported out of the central nervous system and inhibited the efflux of the other, but there was a [125I]P38 preferring subcomponent to the transport system.(ABSTRACT TRUNCATED AT 250 WORDS)