1. Interstitial cells of Cajal (ICs) have been proposed as pacemakers in the gastrointestinal tract. We studied the characteristics and distribution of ICs and electrical activity of small intestinal muscles from mice with mutations at the dominant-white spotting/c-kit (W) locus because the tyrosine kinase function of c-kit may be important in the development of the IC network. 2. W/WV mutants (days 3-30 postpartum) had few ICs in the myenteric plexus region compared with wild type (+/+) siblings. The few ICs present were associated with neural elements and lay between myenteric ganglia and the longitudinal muscle layer. 3. Electrical recordings from intestinal muscle strips showed that electrical slow waves were always present in muscles of +/+ siblings, but were absent in W/WV mice. 4. Muscles from W/WV mice responded to stimulation of intrinsic nerves. Neural responses, attributed to the release of acetylcholine, nitric oxide and other unidentified transmitters, were recorded. 5. These findings are consistent with the hypothesis that ICs are a critical element in the generation of electrical rhythmicity in intestinal muscles. The data also show that neural regulation of gastrointestinal muscles can develop independently of the IC network. 6. W locus mutants provide a powerful new model for studies of the physiological role of ICs and the significance of electrical rhythmicity to normal gastrointestinal motility.