We examined whether signals from rods and S cones can combine to produce a threshold response. Test flashes of specific wavelengths superposed on a long wavelength adapting field were used to isolate threshold responses from the two receptor systems, simultaneously and at the same retinal location. Dark adaptation experiments and spectral sensitivity determinations indicated that, in the adaptational range from about 1.6 to 2.8 log scot td, 530 nm and 440 nm flashes were detected by rod and S cone photoreceptors, respectively. The intensities of the 530 nm and 440 nm flashes were mixed in various ratios and the increment threshold was then measured with these mixture flashes using the method of constant stimuli. The effects of rod and S cone excitation were found to summate linearly at threshold, under these experimental conditions. Summation occurred presumably at an early stage of the visual process.