gamma-Aminobutyric acid (GABA) is an important neurotransmitter that mediates inhibition in the vertebrate CNS. Until recently, two receptor subtypes were known: bicuculline-sensitive GABAA and baclofen-sensitive GABAB receptors. Several lines of evidence now indicate the existence of a third class of GABA receptor, which is distinct pharmacologically from GABAA and GABAB receptors and is found predominantly in the vertebrate retina. These novel GABAC receptors are Cl- pores. They are insensitive to drugs that modulate GABAA and GABAB receptors and are activated selectively by cis-4-aminoacrotonic acid.