Neuropeptides related to ACTH, MSH and LPH are involved in acquisition and maintenance of conditioned behaviour. These peptides affect the behaviour by a temporary selective increase in the state of arousal in limbic midbrain structures, thereby increasing the motivational influence of environmental stimuli. Steroids of adrenal origin affect conditioned behaviour in a way opposite to that of ACTH and related peptides. Such steroids alter the arousal level in limbic midbrain structures to enhance discrimination and consequently the elimination of non relevant behaviourla responses. Neuropeptides related to ACTH play a basic role in motivational, learning and memory processes, while the pituitary-adrenal system through the secretion of corticosteroids has a secondary modulationg function.