Cultured rat hippocampal neurons were classified into three groups on the basis of the functional properties of their AMPA-subtype glutamate receptors. The type I neuron had AMPA receptors with an outwardly rectifying I-V relation and little permeability to Ca2+ whereas the AMPA receptors in the type II neuron were characterized by marked inward rectification and high Ca2+ permeability. In the third type of neuron, the responses of AMPA receptors exhibited intermediate properties in both I-V relation and Ca2+ permeability. We suggest that these intermediate properties in the third type of neuron reflect the coexistence of Ca(2+)-permeant and Ca(2+)-impermeant AMPA receptors.