Delta-opioid receptors (DOR) are highly expressed in the insular cortex, which regulates various autonomic functions including cardiovascular responses. However, the functional roles of DOR in the insular cortex are yet unknown. In the present study we investigated the effects of KNT-127, an agonist of DOR, on the heart rate and neuronal activities in the insular cortex of mice. ddY male mice (5-9 weeks) were used. The electrocardiogram was recorded using wire electrodes inserted subcutaneously in freely moving mice. The extracellular neuronal activities were recorded in the insular cortex in anesthetized mice. The field excitatory postsynaptic potentials (fEPSP) were recorded in the acute insular cortex slices. Microinjection of KNT-127 (46 ng /mouse) in the right, but not left, side of the insular cortex significantly decreased the heart rate. The firing frequency of insular neurons was reduced by intracerebroventricular administration of KNT-127 (230 ng /mouse). Moreover, the fEPSP amplitude was reduced by bath-application of KNT-127 (1-100 μM). Together, activation of DOR in the insular cortex modulates the heart rate via the suppression of neuronal activities.