Intravenous administration of ACTH completely blocks conditioned avoidance reflex activity and the unconditioned escape responses in the rabbit. Inhibition of vasomotor reaction to the conditioned emotional response was also observed. ACTH also decreased blood pressure, both in narcotized and curarized animals, under acute experimental conditions. The effect of ACTH manifested itself also in an inhibition of the spinal flexor reflex. After incubation with NaOH solution and neutralization, intravenous ACTH failed to produce similar changes; but it did prevent the normal inhibitory action of a subsequent injection of non-incubated ACTH.