Thermal trauma caused a distinct increase in content of cAMP and cGMP in blood as well as in liver and kidney tissues of one-month-old rats within the first week after burns. Alterations, which depend on the step of burn development, were found in basal and adrenaline, fluoride-stimulated activities of adenylate cyclase in liver and kidney tissues of experimental animals. Activity of phosphodiesterase was altered only slightly under similar experimental conditions while pronounced decrease in the enzymatic activity was observed in rat liver tissue within 6 hrs after burns.