Three psychophysical paradigms have shown that moderately high retinal illuminance (ca. 1000 rd) long-wavelength chromatic adaptation results in a disproportionate reduction in visual sensitivity to rapidly modulated lights. (i) Critical-flicker-fusion - log retinal illuminance (CFF-log I) functions are shallower for long-wavelength than middle-wavelength modulated lights. (ii) A long-wavelength light modulated just above CFF appears as flickering ifa steady middle-wavelength pedestal is added (iii) The R/C ratio for heterochromatic modulation photometric (HMP) equality increases with light level when the mean chromaticity is approximate to 605 nm, but not when it is approximate to 570 nm. The goal was to evaluate whether Magnocellular - (MC-) pathway retinal ganglion cells show reduced modulation sensitivity with long-wavelength adaptation, as is observed with these three psychophysical paradigms. In Experiment (I), CFFs for 554 and 638 nm lights were estimated by noting the temporal frequencies at which the cells firing fell below 10 imp/s. CFF was measured for light levels between 1-1000 td. Seventeen of 17 units showed a higher CFF at 1000 td for 554 nm, consistent with psychophysical observation. In Experiment (2), a steady 1000 td 554 nm pedestal was added to a 1000 td 638 nm modulated light. Four of 6 units showed higher 638 nm CFF when the steady 554 nm pedestal was present. In Experiment (3), Heterochromatic Modulation Photometry responsivities were obtained at 200 and 2000 td as a function of the relative modulation of 554 and 638 nm counterphase lights. Chromaticity was metameric to either 605 or 570 nm. Fifteen of 17 units showed substantial changes in log (R/G) with luminance for the 605 nm, but not for the 570 nm adaptation condition. Thus, temporal phenomena observed psychophysically may be seen in MC-pathway cells. The results are interpretable as arising front interactions between signals originating from the L- and M-cone receptor types, not from differences in temporal properties of the two-cone types. (C) 2000 John Wiley & Sons, Inc.