This study was designed to examine the propagation and propagation failure of the slow wave. Spontaneous slow waves and action potentials were recorded from the longitudinal muscle layer of small strips of cat jejunum. Slow waves could be stimulated electrically at intervals shorter than the spontaneous interval, and the velocity of propagation of these slow waves was slower than the spontaneous slow waves. The velocity of propagation was approximately proportional to the time between slow waves. At intervals about one-half the normal spontaneous interval the propagation became slow and unstable, which would result in the periodic failure of a single slow wave. This type of failure may underlie the formation of the slow-wave frequency gradient observed in the intact intestine. The wave shape of the external recording of the slow wave, together with the measurement of electrotonic current spread in strips of isolated muscle, suggest that the slow wave propagates by local circuit currents in an electrically excitable cablelike tissue.