Colour vision can be assessed by examining the colour contrast threshold along various colour axes. We investigated the possibility of determining these thresholds objectively by means of visual evoked cortical potentials. A colour-calibrated flicker-free (112-Hz) monitor and a 14-bit-per-gun board allowed visualization of colours with specified xyY CIE 1931 coordinates. Horizontal grating, 1 c/deg were sinusoidally alternated at 8 Hz for both visual evoked cortical potential recordings and psychophysical determinations. Two healthy emmetropic 35 year-old subjects performed colour brightness matching along each colour axis, before any recording and reduction in colour contrast. For each colour axis, extrapolation to zero voltage of the visual evoked cortical potential amplitude versus log colour contrast response allowed determination of the colour contrast threshold. The visual evoked cortical potential-derived threshold changed considerably with the colour axis, with evident intersubject differences. These differences were similar to those observed in the psychophysically determined thresholds. Visual evoked cortical potential responses to suitable chromatic stimuli allow determination of colour contrast thresholds that correspond well to those determined psychophysically. Hence, with the visual evoked cortical potential, accurate objective assessment of colour vision is feasible and may be useful in both research and clinical settings.