1. Transmembrane potentials were measured in smooth muscle cells in the walls of small arteries in the frog tongue and skin of the lateral abdomen. Resting potentials had mean values of 64.7 and 43.6 mV respectively, and action potentials of up to 60 mV amplitude and 100-200 msec duration could be elicited by electrical stimulation in the tongue vessels.2. Micro-electrodes were inserted into smooth muscle cells in the walls of small arteries and arterioles of the rat mesenteric circulation. Intracellular potentials varied continually and maximum polarization averaged 39.4 mV.3. Rhythmic slow waves with a period varying from 4.5 to 7.8 sec were observed, and their amplitude but not their frequency was affected by the level of anaesthesia.4. Spike potentials were recorded, amplitude up to 35 mV, with occasional overshoot, and duration 40-50 msec; these were followed by a positive after-potential, 100-150 msec duration and amplitude up to 8 mV.5. Experiments using chemical and surgical means of interfering with the autonomic nervous system showed that the amplitude, but not the frequency, of the slow waves and the occurrence of action potentials were markedly affected by nervous influences.6. Stimulation of the smooth muscle by asphyxia, electrical stimulation of the splanchnic nerves, and local application of adrenaline, noradrenaline and vasopressin increased the frequency of action potentials and the amplitude of the slow waves, and inhibiting agents such as acetylcholine and removal of the nerve supply depressed the electrical activity.