Properties of spontaneous electrical activity were investigated in smooth muscle tissues isolated from the stomach antrum of humanely killed guinea-pigs. Recording of electrical responses from smooth muscles using intracellular microelectrode reveals that there are three types of electrical response; pacemaker potentials, follower potentials and slow waves recorded from myenteric interstitial cells of Cajal (ICC-MY), longitudinal muscle and circular muscle, respectively (Dickens et al. 1999). In isolated circular muscles with no attached ICC-MY, slow potentials are also produced periodically (Suzuki & Hirst, 1999), as a result of summation of unitary potentials generated in intramuscular interstitial cells of Cajal (ICC-IM; Edwards et al. 1999). Pacemaker potentials are consisted of two components, a primary component with rapid and transient depolarization and following plateau component. The primary component may be formed by activation of Ca²⁺-permeable channels, and the plateau component may be formed by activation of Ca²⁺-activated chloride channels. Slow potential is sensitive to DIDS, suggesting that this potential is formed by activation of Ca²⁺-sensitive chloride channels. Attempts were made to compare the properties between slow waves and slow potentials. Both potentials are recorded from circular muscles, and slow waves are composed of electrotonic component of pacemaker potentials and superimposing slow potentials, while slow potentials are formed by summation of unitary potentials generated in ICC-IM. Both potentials are formed primarily by summation of unitary potentials, and caffeine (1 mM) blocks only slow potentials but not slow waves, suggesting that the mechanism for generating spontaneous activity differs between ICC-MY and ICC-IM. Slow waves are more regular than slow potentials, and the frequency of slow waves is much higher than slow potentials. However, the frequency of the activity is increased by depolarization of the membrane with high-K⁺ solutions or with current injection in both potentials, suggesting that the level of intracellular Ca²⁺ concentrations is one of the key factors for the regulation of frequency. Inhibition of either Ca²⁺-ATPase at the internal stores by CPA, IP₃ receptors by 2-APB, or mitochondrial proton transport by CCCP results in the blockade of slow potentials but not slow waves. In slow waves, lowering temperature increases the duration and decreases the frequency of slow waves, with no alteration to the amplitude. In slow potentials, reduction in temperature again increases the duration, with no change in the amplitude and frequency. The frequency of these spontaneous activities is decreased by KCN, a metabolic inhibitor, suggesting that mitochondrial activity may be causally related to the generation of rhythmic activity. It is suggested that mitochondria take central role for the generation of rhythmic activity in stomach smooth muscle tissues.