Five day old dark-grown seedlings of Secale cereale variety "coup;ar" p:rown at 20 C were subjected to hardening at 2 C "!.n daily increments from 0 to 7 days, to temperature stress at -5 C for 0, 1, and 3 days, and to recovery at 20 C for 0, I, and J days. Unhardened plants were killed by temperature stress but as the time of hardening increased fewer plants were killed. After 5 days of hardening all plants survived subsequent freezing stress. Mitochondria were isolated from the plants after they received various combinations of the three temperature treatments. There were 8 hardening levels, 3 stress levels, and 3 recovery levels giving a total of 8x3x30 72 treatments. Mitochondria from unstressed plants exhibited steady ADPIO ratios, RC ratios, and rate of state 3 respiration over the whole range of hardening and recovery times. Mitochondria from stressed plants were totally inactive with 0 or one day of hardening but as the time of hardening increased so did ADP: O ratio, R~ ratio, and ratp of state J respiratIon. After 5 days of hardenjng mitochondrla from hardened then stressed plants reached a maxImum level of activity. Evidently there was some change 1n the mttochondria during the hardening phase.