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Inhibitory junction potentials (IJPs) in response to single or repetitive stimulation were recorded intracellularly from smooth muscles of the guinea-pig duodenum.The amplitude of the IJP was dependent on the stimulus intensity and duration.The hyperpolarization of up to 30 mV was evoked by a single pulse. The mean values of the latency, the fall time, the rate of hyperpolarization, the half decay time and the time constant for decay of the IJPs evoked by 0.3 msec stimulation were 67 msec, 130 msec, 86 mV/ sec, 156 msec and 194 msec, respectively.The latency and the half decay time were slightly decreased by changes in the stimulus intensity and duration.When repetitive stimulation was applied at low frequencies, successive IJPs were evoked but their amplitudes were decreased gradually.The maximum amplitude of the hyperpolarization was increased with increasing the frequency of stimulation over than 10 Hz. The spike activity due to rebound excitation was potentiated at high frequency. Over 90% of the cells exhibited only the IJP without atropinization. The excitatory junction potentials and the complex type in response to a brief pulse were recorded from a few cells. These results suggest the predominant nonadrenergic inhibition on the duodenal smooth muscles.-inhibitory junction potential; duodenal smooth muscle; rebound excitation