A majority (24/32) of the extracellularly recorded dorsal hippocampus field CA1 putative GABAergic interneurons were excited in conjunction with theta activation on formalin injection (5%, 0.05 ml, s.c. into right hind-paw) in urethane (1.0 g/kg, i.p.)-anaesthetized rats. An increase in activity was observed to the 10th minute (n=24) and also at later time-periods at which a few of the neurons were recorded following injection of formalin. The mean peak increase in activity within 5 min of formalin injection was 6.43+/-0.81 Hz over the average background activity for these neurons (6.46+/-1.04 Hz). Of 24 neurons, 14 exhibited an increase in activity which was rhythmically modulated with theta. With a concurrent administration of formalin and morphine (5 mg/kg, i.p.), the presumed interneurons recorded displayed an initial increase in discharge rate (mean peak increase within 5 min of 6.95+/-1.10 Hz) which then declined with a decrease in theta activity. The effect of concurrent morphine was naloxone reversible. Morphine administration alone resulted in an immediate decrease in the interneuronal firing rate. In presence of the medial septal region lesions, formalin did not evoke an excitation of intemeurons or theta activation. Further, such lesions prevented the decrease in intemeuron activity to morphine administration. The above data are consistent with the notion that (i) the field CA1 interneurons participate in a noxious stimulus-induced and medial septal region mediated pyramidal cell suppression, and (ii) morphine affects CA1 nociceptive responses partly in a fashion consistent with the effect of the drug on septohippocampal neural network processing.