The aspartate analog 2-amino-3-phosphonopropionic acid (AP3) antagonizes glutamate-stimulated phosphatidyl inositide hydrolysis in brain slices, but is reportedly weak or ineffective in antagonizing the phosphatidyl inositide-coupled cloned metabotropic glutamate receptors 1 alpha and 5. Thus we examined the pharmacological properties of AP3 on mGlu1 alpha and mGlu5 receptor responses in Xenopus oocytes. DL-AP3 antagonized mGlu1 alpha and mGlu5 responses, but antagonism was overcome at high glutamate concentrations consistent with competitive inhibition (IC50 = 2.1 mM for mGlu1 alpha). Both responses were also inhibited by (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG). We conclude that the available antagonists cannot distinguish between the mGlu1 alpha receptor and mGlu5 receptor, and that antagonism by AP3 may be obscured in the presence of high agonist concentrations or in cells with spare receptors.