1. The rho 1 protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of gamma-aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABAA receptors, suggested that it may react atypically to other GABA agonists and antagonists. 2. cDNAs for the rho 1 and the alpha 5 beta 1 receptors for GABA were expressed as homo- and heterooligomers, respectively, in Xenopus oocytes. The selectivities of the respective receptors for various agonists were investigated using concentration-response experiments in voltage clamped cells. 3. The most potent agonists at the rho 1 receptor were trans-4-aminocrotonic acid (TACA) > GABA > muscimol; at the alpha 5 beta 1 receptor the rank order was muscimol > GABA > 4,5,6,7-tetrahydroisoxazole[4,5-c]pyridine-3-ol (THIP). The most specific agonists were cis-(2-(aminomethyl)-cyclopropyl-carboxylic acid (CAMP) and THIP for the rho 1 and the alpha 5 beta 1 receptors, respectively. 4. Comparing GABA, TACA and cis-aminocrotonic acid (CACA) at rho 1 receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of rho 1 seems independent of the expression system. 5. Agonists THIP, piperidine-4-sulphonic acid (P4S), and isoguvacine, whose C-C-C-N chains are constrained by rings into a folded conformation and were potent at the alpha 5 beta 1 receptor, were among the weakest at the rho 1 receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the alpha 5 beta 1 receptor but moderately potent at the pl receptor. These findings suggest that the rho l receptor recognizes agonists in the extended conformation, in contrast to GABAA receptors, which are believed to recognize agonists in the partially folded conformation.6. In contrast to the alpha 5 beta 1 receptor, gradations in maximum responses were apparent in the rho l receptor,suggesting various degrees of partial agonism. In particular, imidazole-4-acetic acid (I4AA), whose maximum response was only 3% of GABA's maximum, had an apparent Kd for activating the rho l receptor of 16 microM; but it had an apparent Kd for competitively blocking the receptor of 0.64 microM. This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor.7. Hill coefficients approached 2 at the rho l receptor, but were closer to unity at the alpha 5 beta 1 receptor. Thus,the rho l receptor displayed higher cooperativity.8. Unlike typical GABAA receptors, the rho l receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)-tubocurarine.