Mast cells are relatively common in human omental veins where they generally occur in the tunica adventitia and among the peripheral smooth muscle cells of the tunica media. The distance between mast cells and adjacent muscle cells is often as short as 1 µm. The tunica intima does not contain mast cells. With few exceptions the typical mast cells were oval in shape, had numerous slender projections and contained granules (0.3–0.7 µm) with characteristic stacks of lamellae and scrolls. A few elongated cells with processes containing granules filled with dense homogeneous matrix material were also detected and could represent so-called chromaffin mast cells. Noradrenergic axons and terminals identified by their large (85 nm) and small (50 nm) dense-cored vesicles are present in the vicinity of many mast cells which could allow various interactions between the two cell types and their released amines and ATP. Electrical field stimulation which affects the nerve terminals does not change the ultrastructure of the mast cells unless the α-blocking agent phentolamine (7.5 × 10<sup>-7</sup> M) is present during the stimulation, when degranulation results.