Changes in the amplitudes of the a-wave, b-wave and wavelets of the electroretinogram of the isolated rat retina were observed as a function of alterations in thei onic composition of media bathing isolated rat retinas. All the components of the electroretinogram were reduced in low sodium solutions. An increase of the K + concentration from 5 to 20 mM increased the amplitude of the b-wave but reduced the amplitudes of the a-wave and wavelets. Above 30 mM K + the b-wave was also reduced. Low concentrations of Cl − abolished the b-wave and wavelets but did not alter the a-wave. The amplitudes of the a-wave and b-wave were increased while the wavelets were suppressed in Ca 2+-free media. Implications of these ionic effects are discussed in relation to the cellular origins of these potentials.