Chronic experiments were carried out on five dogs with a defensive instrumental reflex (IR) associated with the maintenance of the flexor posture. Was studied the influence of carbocholine bilateral microinjections (0.05-0.10 mcg) into the nucleus accumbens (NAC) on the realization of the IR and on the behavioural differentiation of acoustic signals in a defensive situation. Cholinergic system of the NAC was shown to participate in both the motor and sensory mechanisms connected with the realization of motor responses to defensive and differentiation signals. Activation of the cholinergic system of the NAC led to an "improvement" of behavioural signal differentiation probably due to an increase of attention to significant stimuli. The cholinergic system of the NAC participated in the regulation of voluntary movements. Its influence was mainly excitatory and had unspecific and prolonged character.