[en] Thermal hysteresis of longitudinal relaxation times (T₁) of water protons in hypocotyls of etiolated Vigna radiata and V. mungo seedlings was investigated by pulse nuclear magnetic resonance (NMR) spectroscopy. Various lengths of chilling exposures during a cool-warm cycle between 20 and 0°C (below 10°C, about 4 h) for the T₁ hysteresis measurement did not cause any visible injury symptoms in hypocotyls. However, the profiles of T₁ hysteresis varied as a result of different chilling exposures. The sums of the T₁ ratio (for detail see Introduction) reflecting T₁ prolongation or shortening upon the warming process were a good quantitative index for the extent of T₁ hysteresis, and the wide dispersion of this value ranging on the “minus” side (T₁ prolongation upon warming) suggested the occurrence of a primary response of cells to chilling stress before obvious visible symptoms occur while the T₁ ratio sums on the “plus” side (T₁ shortening upon warming) corresponded to a response of serious visible injury. Therefore, the sums of the T₁ ratio can be used as a non-destructive diagnostic tool for monitoring the primary event of chilling injury when lacking any visible injury symptoms. The data indicate that the critical temperature for the occurrence of primary response for chilling stress was around 7.5°C for V. radiata and 12.5°C for V. mungo