The effects of conditioning stimuli applied to the commissural pathway on the perforant path (PP) evoked EPSP and population spike response were examined in urethane anaesthetized rats. Whereas conditioning stimuli applied to the PP resulted in a short-term potentiation of both the EPSP and population spike evoked by a test pulse to the same pathway (homosynaptic potentiation), only the population spike was potential when the PP test pulse was preceded by a conditioning volley to the commissural afferent. This form of heterosynaptic potentiation, which occurs in the absence of any significant change in the synaptic currents generated by the perforant path, suggests that alterations in postsynaptic excitability may underly the process of short-term potentiation rather than an augmented transmitter release.