A technique is described which reveals the operation of sustained and transient mechanisms in simple reaction times (RTs) elicited by suprathreshold contrast. responses to grating stimuli with either fast (instantaneous) or slow (cosinusoidally ramped) onsets are compared RTs to chromatic (red-green) stimuli are relatively unaffected by mode of onset over a wide range of suprathreshold contrast and the fast RT/contrast function is monophasic. The achromatic high-contrast RTs show similar independence from mode of onset but, at low contrasts (below about 0.05), the difference between RTs to fast and slow onsets suggests that the underlying mechanism is transient. This is reflected in the biphasic shape of the fast achromatic RT/contrast function. The results support the notion that chromatic RTs are subserved by the parvocellular system, while achromatic RTs are dominated by magnocellular units at low contrast and parvocellular units at high contrast. (C) 2000 John Wiley & Sons, Inc.