The pedunculopontine tegmental nucleus (PPTg) is assumed to be one of the brainstem sites for eliciting the theta rhythm that is expressed in rat hippocampal and occipital EEG in high arousal states. Unilateral lesions of the PPTg in rats induced a hemispheric asymmetry in the expression of occipital theta waves during exploratory sniffing with a significantly lower mean power in the lesioned hemisphere. Acetylcholine was found to be the main neuro-transmitter used by the PPTg neurones. Therefore, the present study was conducted to determine if nicotine was effective in counteracting the lesion-induced hemispheric asymmetry of the theta waves in the occipital EEG of freely moving rats. In 12 rats the PPTg was unilaterally lesioned by ibotenic acid. The occipital EEG was recorded from both hemispheres in three waking states and after intraperitoneal nicotine and physostigmine application. Nicotine reversed the lesion-induced hemispheric asymmetry of the theta rhythm during exploratory sniffing in a dose-dependent manner. Physostigmine did not reverse the hemispheric asymmetry of the theta waves. The present study indicates that a nicotinic component may be involved in the reinforcement or elicitation of the theta rhythm by the pontomesencephalic tegmentum.