1. The pharmacology and ionic regulation of [3H]-2-D-2-amino-5-phosphonopentanoic acid ([3H]-D-AP5) and [3H]-DL-2-amino-7-phosphonoheptanoic acid ([3H]-DL-AP7) binding to homogenates of rat cerebral cortex were examined using radioligand binding methodology. 2. Both [3H]-D-AP5 and [3H]-DL-AP7 labelled a single population of binding sites with dissociation constants of 0.39 and 1.8 mumol/l, respectively. The density of binding sites found with [3H]-DL-AP7 was 13 times greater than that found with [3H]-D-AP5. 3. The ionic requirements of the [3H]-D-AP5 binding site in the presence of chloride were such that calcium acetate enhanced binding, while magnesium and sodium acetate both decreased binding. In the absence of chloride both calcium and chloride ions stimulated binding. 4. In a chloride-free buffer calcium acetate stimulated binding of [3H]-DL-AP7 in a biphasic manner. Chloride ions (ammonium salt) enhanced binding slightly at low concentrations (0.1-1.0 mmol/l) above which binding was reduced to non-specific levels. The ionic dependence of [3H]-DL-AP7 binding had some similarities to the previously defined GLU-C site. 5. The pharmacological profile of the site labelled by [3H]-D-AP5 was consistent with that of a recognition site for N-methyl-D-aspartate (NMDA) as defined in electrophysiological experiments. [3H]-DL-AP7 did not label an NMDA site as several non-NMDA ligands displaced binding with high affinity and the binding was not stereospecific as found for [3H]-D-AP5. Moreover, the pharmacological profile of the [3H]-DL-AP7 site did not correspond to any excitatory amino acid receptor as presently defined.