The brain computes visual colour by analysing the relative excitations of three types of retinal cones1. Each cone excitation is governed by a spectral sensitivity function which relates the amplitude of the neural response to wavelength at constant light intensity. The spectral sensitivities of human cones are not well characterized. We report measuring the sensitivities by recording electrical responses of human cones to stimuli of different wavelengths. Spectral sensitivities of 'green' and 'red' cones, determined over the entire visible region, show peaks near 530 and 560 nm respectively, and are remarkably similar to those of the old-world monkey Macaco fascicularis 2,3. They satisfactorily predict the photopic luminosity function, a measure of the sensitivity of cone-mediated human vision to light of different wavelengths. The kinetics of the light responses of human cones also appeared similar to those of macaque cones: the time to peak response to a dim flash was 50-100 ms and there was a characteristic undershoot during recovery. © 1987 Nature Publishing Group.