Interstitial cells of Cajal (ICC) are believed to lnitlate the basic contractile activity of the gastrointestlnal tract. Interstitial cells of Cajal express c‐kit receptor tyroslne kinase and are deficient in Ws/Ws mutant rats with a small deletion of the c‐kit gene. As Ws/Ws rats show remarkable bile reflux to the stomach, the contraction pressure of the pylorus was compared between Ws/Ws and control +/+ rats. The contraction pressure of the pylorus was measured using a mlcrotransducer, which was Inserted through a pln‐hole in the anterlor wall of the stomach under anesthesla. The magnitude of bile reflux was estimated by measurlng the content of bile acids In the stomach. The c‐kit messenger RNA‐expressing cells were detected by in sltu hybrldlzatlon. Frequency and the maxlmum pressure of the contractlon were comparable between Ws/Ws and +/+ rats, but the duration of the contractlon was significantly shorter In Ws/Ws rats than In +/+ rats. The number of c‐kit messenger RNA‐expresslng ICC in the pylorus of Ws/Ws rats was 1.7% that of +/+ rats. The bile reflux observed in Ws/Ws rats was attributed to the decrease in the duration of the pyloric contraction, which appeared to result from the deficlency of c‐kit messenger RNA‐expressing ICC.