The model of Boynton and Kambe was used to analyze wavelength discrimination functions measured in the fovea by McCree. The long-wavelength portions of these functions, which were measured with several field sizes at three different luminance levels, are described well by the model. Thresholds decrease with increasing field size, but the relationship between threshold and field size is dependent on the color of the lest stimulus. Two factors, summation and opponent-mechanism adaptation, appear to affect the relationship. These two factors can be described with variations in the two coefficients of the Boynton and Kambe equation for red-green discriminations.