An excitatory action of histamine was investigated by intracellular recording in the CA3 region of hippocampal slices. Bath application of histamine or impromidine, a H2 receptor agonist, had the following effects: (1) a depolarisation in 60% and no changes in membrane potential in 40% of the CA3 pyramids; (2) single cell firing and burst activity were evoked or more than doubled when spontaneously present; (3) the bursts were prolonged and often followed by afterdischarges instead of the normal afterhyperpolarisations (AHPs); (4) synaptic stimulation evoked large bursts instead of excitatory synaptic potentials (EPSPs) and primary burst responses became prolonged. CA3 bursts may play a decisive role in memory trace formation, their facilitation and potentiation is in keeping with a positive role of the histaminergic system in attention and learning.