More and more evidence showed that rice plant could grow better if provided in part with nitrate (NO₃^-). Changes of the membrane potential of epidermal cells in root tips of four rice (Oryza Sativa L.) cultivars, i. e., Nongken 57 (Japonica), Siyou 917 (Hybrid Japonica), Yangdao 6 (Indica), Shanyou 63 (Hybrid Indica) were monitored using microelectrodes in different concentrations of NO₃^- (0.1 mmol L^-1, 1 mmol L^-1 and 10 mmol L^-1, respectively). The results were as follows. The absorption of NO₃^- by the roots made the membrane potential depolarized, and then repolarized. The magnitude of depolarization increased with the increasing of NO₃^- concentrations and the average depolarization were 7 mV, 11 mV and 21 mV under the concentrations of 0.1 mmol L^-1 NO₃^-, 1 mmol L^-1 NO₃^- and 10 mmol L^-1 NO₃^-, respectively. Repolariztion appeared 20 minutes after the treatment of 0. 1 mmol L^-1 NO₃^- while it did only after the NO₃^- was withdraw in the treatment of 1 mmol L^-1 NO₃^- and 10 mmol L^-1 NO₃^-. Among the four rice cultivars tested, Yangdao 6 was most sensitive to NO₃^- with its higher level of depolarization and less time to reach the maximum of membrane potential, while Nongken 57 was less sensitive to NO₃^- with its lower level of depolarization and more time to reach the maximum of membrane potential. Siyou 917 and Shanyou 63 exhibited the similar manner on the two aspects. This suggested that Yangdao 6 has the strongest capability to take up NO₃^- while Nongken 57 has the poorest absorption of NO₃^-. Besides, hyperpolarization followed by depolarization was found in parts of rice roots during the absorption of NO₃^-. The results obtained in this experiment implied that response of electropotentials of cell membranes to NO₃^- could be a feasible way to the study of nitrate nutrition in rice plants.