The electrical activity of longitudinal muscle cells of the small intestine of the dog have been recorded in vivo using microelectrodes. This activity is characterized by periodic slow depolarizations of from 3 to 15 mv starting from potentials of 35–50 mv. The frequency of these slow depolarizations is less in the ileum than in the jejunum and is diminished by reduction in body temperature. Asphyxia diminishes both frequency and amplitude of these slow depolarizations without affecting the resting potentials. Action potential spikes arise from the larger slow depolarizations. The records obtained in this study are compared with previously recorded monopolar extracellular records. It is concluded that the slow waves recorded using extracellular electrodes arise from slow depolarizations of intestinal muscle cells. It is proposed that these slow depolarizations are a coordinating mechanism for motility of the longitudinal muscle of the dog intestine. The mechanism of synchronization of the slow waves themselves remains to be elucidated.