The rapid changes in respiratory metabolism that follow exposure of plants grown at cold temperatures to warm conditions have been investigated in barley infected by powdery mildew (Erysiphe graminis f.sp. hordei D. C. Marchal). Total rates of dark respiration, measured between 4 and 20 °C, were higher in cold-grown plants (9 ± 2 °C day, 4 ± 2 °C night) than in warm-grown plants (20 ± 2 °C). Infection caused an increase in respiration at all temperatures. Leaves from healthy cold-grown plants showed a marked change in the contribution of the individual components of total respiration in response to increasing temperature; cyanide-sensitive respiration dropped from 71·2 to 42·8%, while cyanide-insensitive respiration increased from 16·4% to 44·0%, over the temperature range 4−20 °C. Such changes were inhibited by mildew infection. Mlitochondrial respiration exhibited similar trends in response to temperature and infection. Mitochondria isolated from infected cold-grown barley exhibited a higher respiratory control index and a lower ADP:O ratio than those from healthy cold-grown plants. With the exception of a 16% reduction in the fructan fraction in infected barley, there were no significant (P < 0·05) differences in the carbohydrate contents of healthy and infected cold-grown plants. Temperature and infection-induced changes in the pattern of respiration are discussed with respect to plant carbohydrate status and modified mitochondrial efficiency.