Spectral sensitivity curves were measured for foveal test flashes at absolute threshold, and on varying intensities of a blue-white background which stimulates long and medium wave cones about equally. The average data for three subjects could be fitted with a standard deviation of 0.028 log units by assuming independent responses in “ L” and “ C” mechanisms with peak sensitivity near 555 and 605 nm. The C mechanism was adapted only about 86% as much (in log units) as the L mechanism. It is proposed that post-receptoral processes are responsible for this difference in adaptation; the significance of post-receptoral adaptation in photopic and scotopic vision is discussed.