We have measured the chromatic-discrimination ellipses with different luminances for 66 stimuli distributed throughout the CIE-1931 chromatic diagram. The distribution of these stimuli enabled us to analyze the influence of luminance on discrimination from clearly photopic levels to levels that could enter the mesopic range. The results show a clear influence of the luminance level on the areas of the chromatic-discrimination ellipses. These areas remain almost constant when the luminance level is clearly photopic, and clearly increase when the luminance diminishes. However, it is not necessary that the luminance level diminish far below 2-3 cd/m(2) to detect a substantial increase in the area of the ellipses. According to our data, within the photopic range appears a transition interval of luminance above which the most pronounced increase in the area of the ellipse would be appreciable. Other parameters characteristic of discrimination ellipses, such as orientation and relation to semi-axes, vary with the luminance level, although without clearly following any rule. (C) 2001 John Wiley & Sons, Inc.