Searching for the natural ligands interacting with brain excitatory amino acid receptors, we have isolated from cow brain a low-M r ampholyte fraction containing molecules with excitatory properties similar to those of N-methyl-D-aspartate and kainate, which cannot be accounted for by any of the known brain excitants. This finding supports the hypothesis of the existence of excitatory neurotransmitters other than L-glutamate and L-aspartate.