The toxicity of beta-blocker, Befunolol HCl (BFE), to the retinal pigment epithelium was investigated electrophysiologically. Rabbit in vivo ERG b- and c-waves were recorded after intravitreal injections of 0.1 ml of 0.25, 1, 2, 4, 6 and 10% BFE solutions with osmolalities of 304, 340, 383, 432, 493 and 618 mOsm respectively. Phosphate solutions with osmotic pressure corresponding to the BFE solutions were injected as control. In eyes injected with 0.25% BFE, amplitude of either b- and c-waves were unchanged for 60 min, while in eyes injected with BFE with a higher concentration than 1%, the b-waves attenuated but the c-waves enhanced in proportion to the BFE concentrations. In the control group, the b-waves attenuated and the c-waves enhanced by the injection of the high osmotic phosphate solutions up to 500 mOsm, which corresponded to the osmotic pressure of 6% BFE, but attenuated by the phosphate solutions higher than 600 mOsm. Based on the above result, no acute toxicity of BFE to the retina was suggested in dose of its therapeutic use. The attenuation of b-waves by BFE was likely due to the influence of osmotic pressure since no significant difference in the b-waves amplitudes was found between BFE and equimolar phosphate solutions. However, high concentration of BFE itself was suggested to exhibit an effect to enhance the c-wave because a manifest difference was observed in the c-wave amplitudes between the group at higher BFE concentrations and the correspond control group.