A radioimmunoassay (RIA) for the recently discovered crustacean cardioactive peptide (CCAP) has been developed and used to determine contents of CCAP in different parts of the nervous system of the shore crab Carcinus maenas. Immunoreactive material was detected throughout the nervous system. In contrast to the main ganglia which contained low levels of approximately 1.4 pmol CCAP/mg protein (brain and thoracic ganglion), a high concentration was found in a neurohemal structure, the pericardial organs (PO) (868 pmol/mg protein). A predominantly neurohormonal role of CCAP thus suggested is further supported by in vitro release studies. Incubation of POs in high (K+) saline showed that CCAP is secretable in considerable amounts by a Ca++-dependent release mechanism.