The induction of hippocampal theta by reticular stimulation involves a relay to the hippocampus via the posterior hypothalamic-supramammillary region and then the medial septum. Interestingly, sensory- or behaviour-induced theta is accompanied by suppression of hippocampal field CA1 synaptic responses. In the present study, performed on anaesthetized rats, we observed that reticular stimulation also induced a suppression of the CA1 pyramidal cell population spike and the corresponding dendritic field excitatory postsynaptic potential evoked by field CA3 stimulation. This suppression was observed at stimulation intensity below the threshold for generation of CA1 theta and was maximal at stimulation intensities at the threshold for theta. The frequency and amplitude of theta waves, by contrast, increased further with increasing reticular stimulation voltage. Neural inactivation by microinjection of the local anaesthetic procaine (20% w/v, 0.1-0.2 microL) or the inhibitory ligand gamma aminobutyric acid (0.8 m, 0.5 micro L) in the posterior hypothalamic regions, especially the ipsilateral medial supramammillary region, or the medial septum attenuated both the suppression of CA1 pyramidal cell synaptic excitability and theta generation. However, the effects of microinjection on suppression and theta were not always in parallel. Furthermore, the effect of microinjection of gamma aminobutyric acid on reticularly elicited suppression was observed from relatively fewer sites in the posterior hypothalamus as compared with that on theta activation. These results suggest that reticular stimulation evokes an inhibition of CA1 pyramidal cell excitability that (i) is mediated, at least in part, via medial supramammillary and septal regions, but (ii) involves a separate neural pathway from theta generation.