The effects on the hippocampal electroencephalogram (EEG) of intraventricular injections of the nicotinic ligand alpha-Naja naja toxin, and of d-tubocurarine, were studied in rats immobilized with gallamine or anesthetized with urethane. The EEG recordings were taped and processed off-line to calculate power spectra, autocorrelation functions, and averages. In addition, the times at which spike-and-wave complexes appeared were identified and autocorrelation histograms and cross correlations (with the EEG) were made. Naja toxin and d-tubocurarine provoked a 3.5- to 5-Hz theta rhythm in both hippocampi. Higher doses elicited rhythmic epileptic spike and wave complexes which appeared at a preferred phase of theta rhythm. Atropine and medial septal lesions blocked the rhythm and disrupted the rhythmicity of epileptiform activity. We conclude that different neural subsystems sustain the theta rhythm and epileptiform spikes, and discuss the possible mechanisms involved.