Our results show that a morphological correlation exists between catecholaminergic nerve fibers and mast cells in the rat parietal pleura. Mast cells are found in proximity to catecholaminergic nerve fibers. Moreover we have demonstrated that mast cells and nerve fibers in the perivascular areas of rat parietal pleura show the same formaldehyde fluorescence. In rats previously sympathectomized with neurotoxin 6-OHDA the nerve fibers appear disarranged and poorly fluorescent and related mast cells show a low fluorescence. The importance of this correlation and the possible role of mast cells are discussed.