Maize seeds treated with thidiazuron (TDZ) at concentrations ranging from 10^-8 to 10^-11 M were germinated at room temperature until the age of 7 days. The seedlings were then kept for 20 h at 43°C or 3°C (control seedlings at 25°C) followed by a roll-growing culture until 14 days of age, after which the growth parameters and the condition of the cell membranes were studied. It is shown that the synthetic growth regulator TDZ has had a positive impact on young maize plants affected by chilling and heating. The TDZ concentrations most effective in maximizing the thermal resistance of maize seedlings were 10^-10 and 10^-11 M. In the aftereffect of extreme temperatures the growth parameters and membrane permeability in the leaves of maize plants pre-treated with thidiazuron tended to recover to the level of non-stressed plants.