Effects of iontophoretic application of ATP and AMP were tested on glutamate-evoked extracellular spike activity recorded from single neurones in the CA1 pyramidal cell layer of the rat hippocampal slice in vitro. Glutamate, ATP and AMP were applied to the dendritic region of CA1. ATP depressed 11 of 25 units tested; AMP depressed 10 of 18 units tested. The remaining units were unaffected. Typically, the response to either purine developed slowly and persisted after the ejecting current was terminated, full recovery requiring 2-3 min. ATP and AMP were roughly equipotent. All units were tested with control current and none was affected. These results demonstrate a depressant effect of ATP and AMP on glutamate-evoked activity and therefore support earlier evidence of a postsynaptic depressant effect of purines in rat hippocampus.