We measured the sensitivity of macaque ganglion cells to luminance and chromatic sinusoidal modulation. Phasic ganglion cells of the magnocellular pathway (M-pathway) were the more sensitive to luminance modulation, and tonic ganglion cells of the parvocellular pathway (P-pathway) were more sensitive to chromatic modulation. With decreasing retinal illuminance, phasic ganglion cells' temporal sensitivity to luminance modulation changed in a manner that paralleled psychophysical data. The same was true for tonic cells and chromatic modulation. Taken together, the data suggest strongly that the cells of the M-pathway form the physiological substrate for detection of luminance modulation and the cells of the P-pathway the substrate for detection of chromatic modulation. However, at high light levels, intrusion of a so-called luminance mechanism near 10 Hz in psychophysical detection of chromatic modulation is probably due to responses in the M-pathway, arising primarily from a nonlinearity of cone summation. Both phasic and tonic ganglion cells responded to frequencies higher than can be psychophysically detected. This suggests that central mechanisms, acting as low-pass filters, modify these cells' signals, though the corner frequency is lower for the P-pathway than for the M-pathway. For both cell types, the response phase at different frequencies was consistent with the cells' description as linear filters with a fixed time delay.