Among all the enzymes involve din enkephalin degradation "enkephalinase", cleaving the Gly3-Phe4 amide bond, is considered for its specificity with endogenous enkephalins and their receptors. This enzyme, first identified in membrane bound form, has been recently characterized in a soluble one by a new sensitive fluorimetric method substituting the radiometric technique. The possibility to evaluate "enkephalinase" activity in human plasma, amniotic fluid and cerebro spinal fluid (CSF) allows us to investigate its behavior in various physiological and pathological conditions in which alterations of the endogenous opioid system are hypothesized. Our studies were focused on pregnancy, the first period of life, idiopathic headache and opioid addiction. In these conditions "enkephalinase" activity (EKA) generally results increased. In some cases the activity is proportional to the increased amount of substrate, in other cases no correlation seems apparent.