After 48 hours at 2 degrees C, hypocotyls from chill-sensitive Cucumis sativus seedlings showed a burst of O(2) uptake. The alternative pathway became engaged to close to 45% full capacity during this postchilling respiratory burst. However, it only accounted for up to 50% of this increased respiratory O(2) uptake. By 24 hours after chilling, when the seedlings were fully recovered from visible symptoms of chilling injury, the flux through the alternative pathway was back to the level (about 10%) found before chilling. Blocking chilling-induced ethylene production with aminoethoxyvinylglycine had no effect on this increased utilization of the alternative pathway.The direct effects of temperature on respiration rates and the effects of inhibitors suggested that there was a rapid increase in alternative pathway activity and decrease in the cytochrome pathway activity. The possibility that the alternative pathway represents a compensatory mechanism for the more labile cytochrome pathway is discussed.