A study was made of the effect of laser therapy on the content of she biologically active substances serotonin, histamine, PGE2 and PGF2 alpha as well as on the adenylate cyclase system--adenylate cyclase activity and cGMP content. Direct exposure to laser produced an appreciable increase of the content of serotonin and histamine, PGE2 and PGF2 alpha, cAMP and of adenylate cyclase activity. On the healing of ulcerous defect after 5-6 sessions of laser therapy a decrease of the content of serotonin, a rise of histamine, PGE2, PGF2 alpha, cAMP accumulation and adenylate cyclase activation were recorded.