Precipitated withdrawal in rats chronically exposed to delta 9-tetrahydrocannabinol, the major psychoactive principle of the marijuana plant, was unequivocally demonstrated for the first time using a selective antagonist, SR 141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1(2,4- dichloro-phenyl)-4-methyl-1H-pyrazole carboxamide.HCl). This demonstration should provide a powerful stimulus for the systematic study of dependency on the psychoactive cannabinoids.