The response of granule cells and CA1 pyramidal neurones to NMDA was studied in the presence and absence of Mg2+ using an in vitro slice preparation. In the absence of Mg2+ the depolarizing response of hippocampal neurones to NMDA is accompanied by a decrease in input resistance. In the presence of Mg2+ ions, however, the response to NMDA is always associated with an apparent increase in input resistance. These results indicate that the action of NMDA is by a classical mechanism of conductance increase and are in agreement with the suggestion that the apparent increase in input resistance associated with NMDA depolarizations is the result of voltage-dependent channel block by Mg2+ of the NMDA evoked current.