(R,S)-2-Amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid ((R,S)-APPA) is the only partial agonist at the (R,S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) subtype of excitatory amino acid receptors so far described. In light of the pharmacological interest in partial agonists, we have now accomplished the resolution of (R,S)-APPA. (S)-(+)-APPA (5) and (R)-(-)-APPA (6) were obtained in high enantiomeric purity using (R)-(+)- and (S)-(-)-1-phenylethylamine, respectively, as resolving agents. The absolute stereochemistry of 6 was established by X-ray analysis of 6.HCl.0.25H2O. Compounds 5 and 6 were tested electropharmacologically using the rat cortical wedge preparation and in receptor-binding assays using [3H]-AMPA, [3H]kainic acid, and the N-methyl-D-aspartic acid (NMDA) receptor ligands [3H]CPP, [3H]MK-801, and [3H]glycine. Whereas 6 did not significantly affect the binding of any of these ligands (IC50 > 100 microM), compound 5 revealed affinity for only the [3H]AMPA-binding site (IC50 = 6 microM). In electropharmacological tests, 5 showed full AMPA receptor agonism (EC50 = 230 microM). This effect of 5 was insensitive to the NMDA antagonist CPP but was inhibited competitively by the non-NMDA antagonist NBQX (pKi = 6.30). Compound 6, on the other hand, turned out to be a non-NMDA receptor antagonist, inhibiting competitively depolarizations induced by AMPA (pKi = 3.54), kainic acid (pKi = 3.07), and 5 (pKi = 3.57).