Microcuttings of Populus tremula were cultured in vitro for 0 to 15 days at 10°C and compared with cultures kept at 23 °C. The changes in the levels of polyamines were followed in relation to frost resistance (LT₅₀). At 23 °C there was no variation in the level of putrescine; the level of spermidine decreased in the first 5 days and remained relatively stable afterwards. Treatment at 10 °C induced a sharp temporary peaking of putrescine level after 1 day, followed by a transitory decrease. A second peak, higher in duration and intensity, was observed after the 4th day. After a transitory decrease the level of spermidine increased to twice the standard level after 6 days storage. An increase in the freezing tolerance of poplar shoots was achieved when cultures were exposed to 10°C for at least 3 days; however, 3 to 15 days pretreatment at 10 °C increased the freezing tolerance.