Interstitial cells of Cajal (ICC) are responsible for generating electrical slow waves in the gastrointestinal (GI) tract. Slow waves regulate the frequency of contractions of the tunica muscularis, and therefore ICC are critical for normal motility in the small intestine. ICC express Kit, the gene product of c-kit, a protooncogene that encodes a receptor tyrosine kinase. Physiological evidence demonstrating that ICC are pacemakers has come from experiments on W-mutant mice which have few Kit-positive cells at the level of the myenteric plexus (IC-MY) and also lack electrical slow waves. In the past identification of ICC required the use of electron microscopy, however the discovery that ICC express Kit has facilitated studies of the distribution of ICC in several species. Immunoelectron microscopy to relate ultrastructure to Kit expression has only been performed in a limited number of studies of mice. We examined the ultrastructure of Kit-expressing cells in the rat using immunoelectron microscopy and an anti-Kit antibody. We compared the presence and appearance of Kit-expressing ICC in wildtype and Ws/Ws rats, which carry a mutation in the white spotting locus and have a phenotype similar to W/Wv mutant mice. Kit-expressing cells could be detected in the myenteric plexus (MY) and deep muscular plexus (DMP) regions of the small intestine of wildtype animals. In Ws/Ws rats, Kit-expressing cells were not observed in the region of MY, but were observed in the DMP. The density of Kit-positive cells in the DMP of Ws/Ws rats was similar to those in wildtype rats. Electron microscopy showed that Kit-expressing cells at the level of the MY of the rat had similar ultrastructural features as IC-MY in wildtype mice. IC-DMP in the rat of both wildtype and Ws/Ws mutants were similar in structure to IC-DMP of the mouse. We conclude that wildtype rats have IC-MY and IC-DMP in the tunica muscularis of the jejunum. ICC express Kit-like immunoreactivity (Kit-LI) in the rat as in the mouse. IC-MY are absent in the small intestine of Ws/Ws rats, and this corresponds to the lack of Kit-labeling in this region. Ws/Ws rats, however, possess IC-DMP with normal ultrastructural features and Kit-LI. The absence of IC-MY of Ws/Ws rats is likely to account for the abnormal contractile activity of the GI tract observed in these mutants. The present study suggests that Ws/Ws rats could provide an interesting model to investigate the physiological significance of pacemaker activity because they manifest a defect in IC-MY.