(1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and (1R,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (1R,3S-ACPD) were characterized for potency, efficacy, and selectivity at metabotropic excitatory amino acid receptors. 1S,3R-ACPD stimulated [3H]phosphoinositide hydrolysis in slices of the neonatal and adult rat hippocampus with full efficacy and twice the potency relative to what has been shown for (+/-)-trans-ACPD. 1S,3R-ACPD was up to 30 times more potent in activating metabotropic excitatory amino acid receptors, compared to its affinity for [3H]CGS19755 binding to NMDA receptors. In contrast, 1R,3S-ACPD was much less potent, efficacious, and selective than 1S,3R-ACPD. Although 1S,3R-ACPD is not specific, it is a most selective and efficacious agonist at metabotropic excitatory amino acid receptors.