The effects of Co2+ on L-glutamate and L-aspartate responses wee studied in cultured spinal cord neurons of the embryonic spinal cord of the chick by employing the patch-clamp technique in whole cell mode [9]. It was found that Co2+ blocks the responses at negative membrane potentials for both amino acids, while only partial inhibition was observed at positive membrane potentials. Co2+ alone decreases the resting membrane current which exhibits reversal close to zero. It is suggested that the effects of Co2+ are produced by non-specific interaction with negative charges on the outer side of the membrane.