In Parkinson's disease the decrease of dopamine in the nigro-striatal pathway is allied to modifications of other neuromodulation systems. The biochemical disorder of cholinergic, gabaergic and epinephrinergic pathways is present. Moreover the alteration of certain neuropeptides such as endorphins or enkefalins have been found. The Authors analyse the functional repercussions of these important biochemical modifications in the T.I.D.A. tract. In particular the variation of PRL synthesis and secretion due to dopamine deficiency during basal conditions and after pharmacological treatment is discussed.