The delta and theta frequency bands of the electroencephalogram (EEG) in the posterior hypothalamic area (PH) of rats vary according to an ultradian rhythm with a frequency of approximately 1 cycle/100 min. The influence of histamine-related drugs on the ultradian hypothalamic EEG rhythm was now studied in urethane anaesthetized rats. Injected into the lateral ventricle, metoprine (inhibitor of histamine catabolism) and alpha-fluoromethylhistidine (inhibitor of histamine synthesis) did not alter the duration of the rhythmic changes. The H1 receptor agonist 2-(2-aminoethyl)-thiazole was ineffective, while mepyramine (H1 receptor antagonist) prolonged the cycle duration of delta and theta frequency bands. Stimulation of H2 and H3 receptors by amthamine and immepip, respectively, also prolonged the cycle duration of these frequency bands, while the H2 antagonist famotidine and the H3 antagonist thioperamide exerted the opposite effects. Our results indicate that the ultradian EEG rhythm in the PH is susceptible to regulatory influences mediated by the histaminergic system of the brain.