Until recently the metabotropic excitatory amino acid receptor could only be distinguished from ionotropic receptors by the nature of its second messenger system--phosphoinositide hydrolysis. However, the advent of new pharmacological tools, in particular the selective agonist trans-ACPD, has now allowed this receptor to be distinguished pharmacologically. Darryle Schoepp, Joel Bockaert and Fritz Sladeczek analyse the new data which can be correlated to functional responses and linked with physiological and pathological conditions.