It was shown in acute experiments on rats that one hour after an intraperitoneal injection of theophylline (50 mg/kg) there was a decrease in the NAD + NADP content by 19.4%, a tendency to a fall of NAD.H2 + NADP.H2 was expressed, and the total nicotinamide coferment level was reduced. A tendency to decrease NAD + NADP and the total pyridine nucleotide level was seen after caffeine administration. The action of catecholamines and methylxanthines was compared. Theobromine produced no significant effect on the indices under study. It was shown that isadrine decreased the NAD + NADP level; adrenaline (25 mkg/kg) increased the content of both the oxidized (by 24%) and of the reduced (by 48%) forms of pyridine nucleotides. An increase of adrenaline dose to 1000 mkg/kg was accompanied by reduction of the oxidized forms (by 22.2%) and of the total nicotinamide coferment level (by 18%).