Pyramidal neurons in the rat CA1 hippocampal area contain intracellular receptors for the steroid hormone corticosterone, and membrane associated alpha- and beta-adrenergic receptors. We compared the effect of noradrenaline (NA) on the synaptically evoked CA1 field potential in adrenalectomized (ADX) rats, where corticosteroid receptors are not occupied, with sham-operated controls. It appeared that the increase in the amplitude of the field potential induced by 10-mu-M NA was more pronounced in slices from ADX rats than in sham operated controls. In vitro application of 30 nM corticosterone (20 min) to slices from ADX rats shifted the NA-evoked increases in synaptic efficacy towards the level of the sham operated rats. The incidence of a secondary population spike after application of NA was increased in slices from ADX rats compared to slices from sham controls or ADX rats treated with corticosterone. Occupation of receptors for corticosterone in the hippocampal CA1 area may therefore potentially lead to suppression of excitability.