The pharmacological effects of the enantiomers of cis-3-aminocyclopentanecarboxylic acids ((+)- and (-)-CACP), the enantiomers of trans-3-aminocyclopentanecarboxylic acids ((+)- and (-)-TACP), and the enantiomers of 4-aminocyclopent-1-ene-1-carboxylic acids ((+)- and (-)-4-ACPCA) were studied on human homomeric ρ 1 and ρ 2 GABA C receptors expressed in Xenopus oocytes using two-electrode voltage clamp methods. These compounds are conformationally restricted analogues of γ-aminobutyric acid (GABA) held in a five-membered ring. (+)-TACP (EC 5 0 (ρ 1 )=2.7+/-0.2 μM; EC 5 0 (ρ 2 )=1.45+/-0.22 μM), (+)-CACP (EC 5 0 (ρ 1 )=26.1+/-1.1 μM; EC 5 0 (ρ 2 )=20.1+/-2.1 μM) and (-)-CACP (EC 5 0 (ρ 1 )=78.5+/-3.5 μM; EC 5 0 (ρ 2 )=63.8+/-23.3 μM) were moderately potent partial agonists at ρ 1 and ρ 2 GABA C receptors, while (-)-TACP (100 μM inhibited 56% and 62% of the current produced by 1 μM GABA at ρ 1 and ρ 2 receptors, respectively) was a weak partial agonist with low intrinsic activity at these receptors. In contrast, (+)-4-ACPCA (K i (ρ 1 )=6.0+/-0.1 μM; K i (ρ 2 )=4.7+/-0.3 μM) did not activate GABA C ρ 1 and ρ 2 receptors but potently inhibited the action of GABA at these receptors, while (-)-4-ACPCA had little effect as either an agonist or an antagonist. The affinity order at both GABA C ρ 1 and ρ 2 receptors was (+)-TACP>(+)-4-ACPCA (+)-CACP>(-)-CACP (-)-TACP (-)-4-ACPCA. This study shows that the cyclopentane and cyclopentene analogues of GABA affect GABA C receptors in a unique manner, defining a preferred stereochemical orientation of the amine and carboxylic acid groups when binding to GABA C receptors. This is exemplified by the partial agonist, (+)-TACP, and the antagonist, (+)-4-ACPCA.