Using a multiple-site optical recording technique employing a fast voltage-sensitive dye, we found a novel type of gamma-aminobutyric acid (GABA) response, which is insensitive to GABA(A) and GABA(B) antagonists, but is stimulated by either GABA(A) or GABA(B) agonist. This evidence was identified in the early embryonic brainstem slice preparation.