The effects of two cypermethrin insecticides, alpha-cypermethrin and theta-cypermethrin on the transient outward potassium (I A ) channels in freshly dissociated hippocampal CA3 neurons of rat, were studied using whole-cell patch clamp technique. The results indicate that alpha-cypermethrin increased the amplitude of I A and theta-cypermethrin decreased the amplitude of I A , and both of them shifted steady activation curve of I A towards negative potential. Theta-cypermethrin contributed in the inactivation of I A . Alpha-cypermethrin prolonged the time constants for the recovery from inactivation kinetics of I A . These results imply that transient outward potassium (I A ) channels are the target of cypermethrin, which may explain the mechanism of toxical effects of cypermethrin on mammalian animals.