Interstitial cells of Cajal (ICC) have been identified in the wall of the rabbit urethra with c-Kit antibodies (Lyons et al, 2007) and share morphological characteristics with ICC from the gastrointestinal tract. These cells exhibit spontaneous electrical and Ca2+-activity (Sergeant et al, 2000) and may act to control contractility and tone of the urethral smooth muscle. ICC are ultrastructurally distinct from smooth muscle cells and fibroblasts and the aim of the present study was to examine the ultrastructural properties of urethral ICC with transmission electron microscopy. Bladders and urethras were removed from guinea-pigs (n=4), killed by cervical dislocation in accordance with Schedule 1, UK Home Office regulations. The proximal urethra was prepared for transmission electron microscopy using standard protocols. We have previously shown that c-Kit-positive ICC are present within the muscularis of the rabbit urethra (Lyons et al, 2007). In the present study, the muscularis region of the urethra was studied with electron microscopy. Cells with the typical properties of ICC were identified in the circular and longitudinal smooth muscle layers, both on the boundary of the smooth muscle bundles and within the bundles. ICC-like cells were electron dense, contained thin and intermediate filaments, had a basal lamina, contained mitochondria, rough and smooth endoplasmic reticulum, Golgi complexes and membrane caveolae. Unlike the neighboring smooth muscle cells, the ICC-like cells did not contain thick filaments or dense bodies, consistent with the reported inability of ICC to contract. ICC were usually branched and made contacts with up to 10 smooth muscle cells. Gap junctions were not encountered although this type of contact cannot presently be ruled out. ICC were also often found adjacent to nerves and in addition, nerve-smooth muscle cell contacts were frequently observed. Cells with the typical ultrastructural characteristics of ICC (Komuro, 1999) were identified in sections of guinea-pig urethra using transmission electron microscopy. These findings support previous published work on urethral tissue and isolated cells with confocal microscopy.