The inhibition by opiates and the sudden normalization by opioid antagonists of the brain L-asparaginase activity (BAA) have previously been reported to be the main factors in the development of physical dependence and the manifestation of precipitated abstinence syndrome, respectively. As a result, L-asparaginase inhibitors D-aspartic acid and prolyl-leucyl-glycinamide (PLG) were separately given to mice and rats either just after morphine (M)-containing pellet implantation or 15 min before naloxone (NL)-precipitated abstinence syndrome. The animals treated in this manner were used to assess the intensity of the physical dependence and to determine the BAA. D-ASP or PLG administration following pellet implantation significantly increased all of the observed signs such as flying, jumping, wet dog shake and writhing. When D-ASP or PLG were given 15 min before precipitated abstinence they significantly decreased the number of the signs. The determination of the BAA showed significant decreases or increases more or less parallel to the severity of the physical dependence on M. The intensification of physical dependence by D-ASP or PLG given just after the pellet implantation was attributed to their additional inhibitory effect to that of M on the BAA at the beginning of the physical dependence development. The attenuating effect of BAA inhibitors D-ASP or PLG administered before precipitated abstinence was explained with the prevention of the increase in the BAA.