Daily administration of hydrocortisone first increased and then decreased amplitude of multiquantal end-plate currents induced by motor nerve stimulation. The initial facilitating phase of the hormone action was accompanied by elimination of the ATP pre-synaptic effect. Later the inhibitory effect of the ATP becomes restored. The counteraction of the ATP effect was reproduced in the isolated muscle bathed in the saline with hydrocortisone which suggests a non-genomic action of the hormone on pre-synaptic P2 receptor. The data obtained suggest that prevention of the ATP inhibitory action might be a component of a facilitating acute stress reaction.