Simple reaction time was measured to spectral lights matched photometrically in luminance. When these lights were presented on a dimmer achromatic background, reaction time did not vary as a function of wavelength. Moreover, reaction times to white and chromatic lights were the same. When the luminance of the background was the same as that of the chromatic lights, reaction time increased and showed a strong effect of wavelength. Reaction time in this condition appeared to follow a saturation function. The results are described in terms of the operation of achromatic and chromatic processing channels.Previous data on the effect of wavelength on simple reaction time (RT) have been in remarkable accord: when chromatic lights are matched in luminance at photopic levels, and when these lights are presented on a dimmer, achromatic background, simple RT does not vary with wavelength (Finn & Lit, 1971;Holmes, 1926;Lit, Young, & Shaffer, 1971;Mollon & Krauskopf, 1973;Pollack, 1968).Because the chromatic stimuli in these experiments appeared on a less lurninant background, there were actually two cues accompanying stimulus onset on which subjects could base a response. They could respond to the luminance increment or to the chromo atic signal. If they used the former cue, one might not expect a wavelength effect, inasmuch as the size of this luminance increment was matched for all wavelengths.One way to ensure that subjects respond on the basis of the chromatic signal is to use a disjunctive RT task. Jones and Wilkinson (1975) presented subjects with equally luminant white, red, or green stimuli on a dark background. Subjects were instructed to respond only to the chromatic lights and to withhold responses to the white light. They were thus unable to respond on the basis of the luminance signal which was present on every trial. The results showed that RT to green (510 nm) was about 24 msec slower than that to red (645 nm).An alternative way to isolate the effect of color on RT is to match the luminance of the target stimuli and the background so that no luminance change accompanies stimulus onset. When Pieron (1931) measured simple RT under these conditions, he found that RT was shorter to red than to green or blue. However, neither the details of his luminance Some recent studies, using other techniques to measure the effect of wavelength on visual latency, have compared conditions in which the chromatic stimulus is accompanied by a luminance change and those in which it is not. Breton (1977) and Weingarten (1972) found that when subjects made temporal order judgments to chromatic lights presented on a dimmer achromatic background, there was no effect of wavelength. However, when the chromatic stimuli and the background were matched in luminance, the relative latency for red was less than that for green. In Weingarten's data, the difference was 20 to 25msec. Bowen, Lindsey, and Smith (Note I), measuring two-pulse resolution thresholds, found no wavelength effect for chromatic stimuli accompanied by a luminance cue; whe...