Aminophylline and caffeine produced an aug mentation of the twitch response of the isolated phrenic nerve diaphragm preparation caused either by direct or indirect stimulation. Both catecholamine and neostigmine potentiated the effect of xanthine derivatives. The effects of amino phylline and caffeine, both during indirect and direct stimulation, were equally depressed or abolished by nicotinic acid. Cyclic-3’,5’-AMP itself did not affect either the response of diaphragm to indirect or direct stimulation. Both xanthines exhibited an anti-curare effect. The effects of xanthine derivatives and catecholamines were well correlated with the action of glucose-6-phosphate, which by itself produced the same effects as did aminophylline, caffeine and catecholamines.