Excitatory amino acid receptors of the AMPA receptor subtype display a pronounced stereoselectivity for agonists with, in particular, L-AMPA showing 3800 times higher affinity than its D-enantiomer. The observed receptor stereoselectivity may be dramatically affected by the presence of an - often unknown -enantiomeric impurity in the less active enantiomer. Accordingly, pharmacological studies of enantiomers should be performed on compounds with a well defined enantiomeric purity. These considerations are illustrated by the preparation of the enantiomers of the AMPA receptor agonists AMPA and Br-HIBO by enzymatic resolution techniques using chymotrypsin and immobilized amino acylase, respectively, and determination of the enantiomeric purities of the products by chiral ligand exchange chromatography.