Acutely isolated rat hippocampal CA3 pyramidal neurons were irradiated with a semiconductor laser of 650 nm wavelengh and 5 mW power, and properties of delayed rectifier potassium (K+) channel were studied using the whole-cell patch clamp technique. The experiment indicated that low level laser reversibly reduced the amplitudes of IK in a time-dependent and voltage-dependent manner. The percentage of inhibition was up to 34.54%±3.22% (n=15) in irradiating 5 min. The maximum activated current densities of control group, irradiation group and restoration group respectively were 429.78±41.40 pA/pF, 283.26±39.62 pA/pF (n=10, P<0.01) and 397.22±36.81 pA/pF (n=10, P>0.05). Laser irradiation significantly affected the activation process of IK. The half-activation voltage and the slope factor of the activation curves were also changed by the laser′s exposure. The half-activation voltages of control group and irradiation group were 5.74±1.56 mV and 20.98±8.85 mV (n=10, P<0.01) respectively, and the slope factors were 16.51±6.67 mV and 17.44±5.19 mV (n=10, P>0.05) respectively. The results show that low level laser can change the properties of delayed rectifier K+ channel. Therefore, repolarization process of action potential is affected. Further, physiological functions of neurons are adjusted, which might contribute to the restoration and regeneration of injured neurons.