The authors examined the action of fine newly synthesized xanthine derivatives on the activity of the enzyme 3',5' AMP phosphodiesterase (at high substrate concentrations) in homogenates, obtained from the brain lung and myocardium of a rat. The compounds were compared with theophylline for their activity. They showed low inhibiting activity in brain homogenates in contrast to the manifested activity of some of them in lungs and myocardium. Especially manifested difference was observed for the compound with a code 1t which was find times stronger inhibitor in the lung than in the brain. The authors make an inference that some of the compounds manifect selective activity as inhibitors of the enzyme phosphodiesterase and in this sense the search for xanthine derivatives with specific activity is prespective.