Rod photoreceptors express a unique type of noninactivating potassium channels, Kx channels, which play an important role in setting the dark potential and participate in shaping the light response. Biophysical studies of Kx channels are limited. For example, the effects of a conventional blocker of potassium channels, tetraethylammonium (TEA), on Kx channels have not been extensively studied. Here we demonstrate that TEA blocks Kx channels, one molecule of TEA being sufficient to block the channel. Half of the Kx current was inhibited at K0.5 = 5.6 mM. The TEA-induced block of Kx channels depended on extracellular potassium: the higher the potassium concentration, the stronger the block. Using TEA, we blocked potassium channels to reveal their role in shaping the simulated light response (SLR) of rods. We showed that TEA slowed down SLR and sometimes caused generation of action potentials. We developed a complete computer model of the rod, which accurately reproduced the main features of the light response and allowed us to demonstrate that it was suppression of Kx channels that was essential for slowing SLR and increasing excitability of rods. The results reported in this work further establish the importance of Kx channels in rod photoreceptor function.