Leaves of barley cv. Maris Otter, susceptible to powdery mildew, show a large increase in respiration rate when infected with this fungus. About half of the increase is due to increased electron flow through the cytochrome chain and half through the alternative pathway. The alternative pathway shows increased engagement following infection but no increase in capacity. Respiration is not limited either by substrate or supply of inorganic phosphate. While the increase in activity of the cytochrome path may be due to adenylate regulation, that of the alternative path is not understood. The more resistant cvs Athos and Atem do not show alternative path activity following infection with mildew.