Molds of the isolated ileocolonic junctional area were made in 61 dogs (2 conscious, 6 anesthetized, and 53 immediately after death) by the forceful injection into the proximal colon of a mixture of expanding silicone-foam rubbers. An annular constriction (mean width 0.83 cm. S.E.M. ±.067, mean length 0.40 cm. S.E.M. ±.026) was consistently demonstrated on the molds at the junction of small and large intestine. A similar ring-like impression (mean width 1.14 cm. S.E.M. ±.060, mean length 0.31 cm. S.E.M. ±.028) occurred at the point of meeting of cecum and colon. The configuration of the molds at both junctional zones indicates the presence of a narrow band of less distensible bowel at these sites.