During an investigation of excitatory amino acids on cultured embryonic Xenopus neurons, we observed that commercial preparations of glutamine had weak agonist activity on NMDA-type glutamate receptors. Threshold responses were observed at 100 microM glutamine, and the dose-response relation did not show inflection or saturation at concentrations of up to 10 mM. However, NMDA receptor activation induced by glutamine probably represented activity of residual glutamate because: (a) recrystallization of glutamine reduced residual glutamate levels (measured by HPLC analysis) and NMDA receptor activation by comparable amounts; and (b) glutamate at concentrations close to those predicted to be present in glutamine preparations elicited currents of similar amplitudes. Our data indicate that residual glutamate at levels of less than 0.05% are sufficient to confound studies of weak NMDA receptor agonists.