Cold hardiness of pine needles and apple bark was determined in a series of experiments. Plants were exposed to 0°C and 20°C and exposed to day length conditions of 9 and 14 h. All 16 possible combinations were investigated by transfer of the plants from the original condition to each of the other conditions. In order to interpret the results, it was necessary to distinguish between a single and a dual environmental factor change. In pine plants at 20°C, a change in two factors decreased cold hardiness contrary to a single change in temperature or day length. In plants at 0°C, the effects of a change in day length and temperature were additive. Differences in pine and apple are discussed in relation to natural conditions and to geographical distribution. It is suggested that for increased hardiness shortening of day length should precede a low temperature regime and that reversal of this order may upset the signal system, resulting in dehardening.