Intracellular recordings were obtained from pedunculopontine tegmental nuclei neurones in guinea pig brainstem slices. These neurones were characterized by the presence of a slow regular firing (around 3 spikes/s), a broad action potential (more than 1 ms) and a transient rectification indicating the presence of an A current. Bath-application of histamine at 10(-4) or 10(-5) M induced a reversible increase in spontaneous firing. In presence of tetrodotoxin (1 microM), the effect of histamine was a reversible membrane depolarization. It was a direct effect as it persisted in presence of a low calcium/high magnesium solution. This excitatory action was presumably mediated by histamine H1 receptors as it could be blocked by the H1 receptor antagonist mepyramine but not by the H2 receptor antagonist cimetidine. A role in arousal is discussed.