Recently, the measurement of chlorophyll a fluorescence has been applied extensively to detect environmental stresses in plants. However, methods of measurement applicable to higher plants at room temperature have not been fully developed. The present study was conducted to analyze the factors affecting the measurement of chlorophyll a fluorescence in cucumber plants. The results suggested that higher values of Fv/Fm could be obtained by using lower intensities of actinic light. The light intensity irradiated on plants immediately before dark adaptation influenced the Fv/Fm values of chlo rophyll a fluorescence. However, a period of 2 hr dark adaptation was long enough at a photosynthetic photon flux below 200 μmol/m2/s. Plants grown at 25℃ showed the largest value of Fv/Fm compared with those grown at 15 and 35℃. Since differences in plant growth temperature may affect the value of Fv/Fm, it is likely that the dark adaptation period required to obtain stable values of Fv/Fm depends on the light intensity during plant growth, and not on the temperature. Leaves with partial dark adaptation showed almost the same values as leaves with full dark adaptation. It appears that par tial dark adaptation with an attachment for the fluorometer may enable to detect stresses in plants in open field under sunshine conditions.