The present study was designed to investigate i) the action of various nucleotides on membrane permeability of rat red blood cell and hepatocyte for Na⁺ and Rb⁺ ii) the characteristics of purinoceptors on these cell membranes. Blood from Sprague-Dawley rats was obtained by carotid arterial cannulation. Red blood cells were then washed 3 times with saline at 4˚C. Hepatic parenchymal cells were isolated from rat livers by using a modification of the Berry and Friend (1969) method. For the Na⁺ influx studies, isolated RBC and hepatocyte were incubated in incubation medium containing ²²Na⁺0.2 μCi/ml at 37℃. After various time intervals samples were removed from the incubation flask and washed out 3 times with ice-cold washing solutions. Cells were destroyed by adding Triton X-100 and TCA solution. After centrifugation, the supernatants were assayed for ²²Na⁺ by gamma counter. ⁸⁶Rb⁺ was used to simulate K⁺ in these K⁺efflux studies. Isolated hepatocytes were incubated for 60 min in the loading solution containing ⁸⁶Rb⁺ 10 μCi/ml at 37℃. After loading, the cells washed out 3 times by centrifugation with washing solution. The cells were incubated in buffer solution at 37℃. At intervals thereafter, samples were removed and centrifuged. The supernatants were analyzed for ⁸⁶Rb⁺ by liquid scintillation counter. The main results of the experiments were: 1) ATP and ATPP increased in both ²²Na⁺ influx and ⁸⁶Rb⁺ efflux in the red blood cell. Although ADP showed a tendency to increase in RBC membrane permeability for ²²Na⁺ and ⁸⁶Rb⁺, the changes were not significantly different from the control. 2) The Significant changes in ²²Na⁺ and ⁸⁶Rb⁺ flux by ATP were also demonstrated in hepatocyte. ATPP and ADP showed a tendency to increase in hepatocyte membrane permeability for both ions. 3) Other nucleoside triphosphates-ITP, GTP and CTP-did not change in membrane permeability for ²²Na⁺ and ⁸⁶Rb⁺ in RBC and hepatocyte. In conclusion, not only ATP but also ATPP activate purinoceptors and change in membrane permeability for Na⁺ and K⁺. In order to activate purinoceptors on the cell membrane, the nucleotides have to possess intact adenine moiety and three phosphates or more in its molecule.