The reaction of wavelength-selective (WL), wavelength-opponent (WLO) and colour-coded (CO) cells in monkey visual cortex to changes in the wavelength composition of the light reflected from the area in their receptive fields was studied, using multicoloured displays. Wavelength-selective and wavelength-opponent cells were found to be very sensitive to changes in the wavelength composition of the light reflected from the areas in their receptive fields, irrespective of their perceived natural and void colours. Changes in the wavelength composition of the light reflected from surrounding areas did not affect their responses. They were also sensitive to the order in which lights of various wavelengths illuminated the areas in their receptive fields. Colour-coded cells were not affected by changes in the wavelength composition nor were they sensitive to the sequence with which the area in their receptive fields was illuminated by lights of different wavelengths. However, they required that the display, with the area of their preferred colour in their receptive fields, be trichromatically illuminated. This and other evidence suggested that such cells were sensitive not only to the illumination of the area in their receptive fields, but of surrounding areas as well. This evidence reinforces further the distinction between wavelength-selective and colour-coded cells and leads to the conclusion that one function of the wavelength-selective cells must be to register the changes in wavelength composition which occur throughout the day.