When the velocity of capillary blood flow in the frog tongue declined to an intermediate range of 0.2-0.7 mm/s, the glossopharyngeal nerve stimulation induced a biphasic slow depolarizing and slow hyperpolarizing potential (HP) in taste cells. The objective of this work was to examine the generative mechanisms of the biphasic slow potentials. The biphasic slow response was always preceded by a slow depolarizing potential (DP) component and followed by a slow HP component. Intravenous injection of tubocurarine completely blocked the biphasic slow responses, suggesting that both components of the biphasic slow potentials are evoked by the parasympathetic nerve (PSN) fibers. Membrane conductance of taste cells increased during slow DPs and decreased during slow HPs. The reversal potential of either component of a biphasic slow response was the almost same value of -12 mV. An antagonist, L-703,606, for neurotransmitter substance P neurokinin(1) receptor completely blocked both components of the biphasic slow responses. An antagonist, flufenamic acid, for nonselective cation channels on the taste cell membrane completely blocked the biphasic slow responses. These results suggest that PSN-induced biphasic slow responses are postsynaptically elicited in taste cells by releasing substance P at the PSN axon terminals. It is concluded that the slow DP component may be generated by opening one type of nonselective cation channel on taste cells and that the slow HP component may be generated by closing the other type of nonselective cation channel. We discussed that a second messenger inositol 1,4,5-trisphosphate might be related to a slow DP component and another second messenger diacylglycerol might be related to a slow HP component.