Thirsty dogs were learned to regulate the supply of water by flexing the paw. The movement was effective only during an acoustic stimulation, either immediately at its beginning, or 10 to 20 sec. after its isolated action. The acoustic signal produced both flexion of the paw (instrumental reflex) and rhythmic swallowing movements which preceded the moment when water was fed into the mouth (classical conditioned reflex). The effect of prolongation of the isolated acoustic stimulation was manifested with extinction of the classical conditioned reflex and activation of the instrumental one.
