Glucocorticoids modulate brain function and behaviour through different mechanisms. Although classical effects are mediated through intracellular receptors that modulate gene transcription, recent evidence supports the existence of rapid, nongenomic steroid effects through the neuronal membrane. In this study, we explored possible rapid behavioural effects of corticosterone in the rat, which could provide a model to characterize further the mechanisms involved in rapid corticosteroid nongenomic actions. We found that a corticosterone injection, at doses (2.5 or 5 mg/kg) that mimic plasma concentrations produced by substantial stress, rapidly increases (within 7.5 min of its systemic administration) the locomotor response displayed by rats in a novel environment (activity cage). A lower dose of 1 mg/kg failed to induce this effect. In addition, corticosterone failed to increase locomotion when administered to rats that had been previously exposed to the activity cage. Corticosterone-induced increased locomotion in a novelty situation was not counteracted by either the intracerebroventricular administration of the protein synthesis inhibitor cycloheximide, or by the intracerebroventricular administration of specific antagonists for each type of intracellular corticosteroid receptor, i.e. RU28318, a mineralocorticoid receptor antagonist and RU38486, a glucocorticoid receptor antagonist. Further studies supported the viability of the receptor antagonists to display an anti-corticosteroid action interfering, as previously reported, with the behavioural &winning test. Therefore, the rapid actions of corticosterone in locomotor activity described here, which appear to be nongenomic, might provide a model for future research on the elucidation of the mechanisms involved in steroid-membrane interactions.