Intracellular recordings were made from cells located in the longitudinal, inner and outer circular muscle layers of the dog, cat, rabbit, opossum and human small intestine. In whole-thickness preparations in all five species, longitudinal muscle cells generated slow waves and spikes. However, in isolated longitudinal muscle preparations, all cells tested were electrically silent. In whole-thickness and in isolated preparations, cells in the inner circular muscle layer generated spontaneous spikes superimposed on slow potentials. However, the occurrence of spikes and slow potentials was more regular in whole-thickness preparations. In whole-thickness preparations, cells in the outer circular muscle layer generated slow waves which were coupled with phasic contractions. However, in isolated outer circular muscle preparations, all cells tested were electrically silent and spontaneous phasic contractions were absent. In whole-thickness preparations, non-neural cells located on the serosal side of the outer circular muscle layer generated slow waves. The data suggest that spontaneous slow waves of the small intestine of the dog, cat, rabbit, opossum and human are generated in non-neural cells located between the longitudinal and outer circular muscle layer and by non-neural cells located between the outer and inner circular muscle layers.