The present study uses simple reaction time (RT) to examine the temporal response to chromatic changes of the red-green (L-2M) and yellow-blue (S-(L+M)I chromatic opponent channels. The hue-substitution method,vas adopted to negate the luminance channel and generate changes in the chromatic input. The equiluminant stimuli were distributed along three tritanopic and three red-green confusion lines. RTs were measured at equiluminance from three reference stimuli with chromatic changes occurring in either the tritan or red-green directions. These reference stimuli also allowed evaluation of the influence of chromatic adaptation level on RT The results show ed that the mean value of RT, for the cone-input variation either of the L-2M channel (S-constant cone axis) or of the S-(L+M) channel (L&M-constant cone axis), was described by a similar law as for luminance variations (Pieron's law). Then, the sensitivity of the two channels for chromatic changes lr us equalized to study their relative response speed in addition, for both axes, an increase in the chromatic adaptation level lengthens response delay.