1. Gamma-aminobutryic acid (GABA), a major inhibitory transmitter of the vertebrate retina, is synthesized from glutamate by L-glutamate decarboxylase (GAD) and mediates neuronal inhibition at GABAA receptors. GAD consists of two distinct molecular forms, GAD65 and GAD67, which have similar distribution patterns in the nervous system (Feldblum et al., 1990; Erlander and Tobin, 1991). GABAA receptors are composed of several distinct polypeptide subunits, of which the GABAA alpha 1 variant has a particularly extensive and widespread distribution in the nervous system. The aim of this study was to determine the cellular localization patterns of GAD and GABAA alpha 1 receptor mRNAs to define GABA- and GABAA receptor-synthesizing neurons in the rat retina. 2. GAD and GABAA alpha 1 mRNAs were localized in retinal neurons by in situ hybridization histochemistry with 35S-labeled antisense RNA probes complementary to GAD67 and GABAA alpha 1 mRNAs. 3. The majority of neurons expressing GAD67 mRNA is located in the proximal inner nuclear layer (INL) and ganglion cell layer (GCL). Occasional GAD67 mRNA-containing neurons are present in the inner plexiform layer. Labeled neurons are not found in the distal INL or in the outer nuclear layer (ONL). 4. GABAA alpha 1 mRNA is expressed by neurons distributed to all regions of the INL. Some discretely labeled cells are present in the GCL. Labeled cells are not observed in the ONL. 5. The distribution of GAD67 mRNA demonstrates that numerous amacrine cells (conventional, interstitial, and displaced) and perhaps interplexiform cells synthesize GABA. These cells are likely to employ GABA as a neurotransmitter. 6. The distribution of GABAA alpha 1 mRNA indicates that bipolar, amacrine, and perhaps ganglion cells express GABAA receptors having an alpha 1 polypeptide subunit, suggesting that GABA acts directly upon these cells.