A hyperbolic equation of the form (R/Rmax) = Ln/(Ln + Kn) has been used to describe the b-wave luminance-response function of the dark-adapted human electroretinogram (ERG). However, this equation accurately describes the function only at low to moderate flash luminances. At high flash luminances, a second amplitude increase or "limb" appears in the function. The results of the present study demonstrate that this limb does not represent a cone system response. First, the spectral sensitivity of the entire luminance-response function is rod-determined in normal subjects. Second, the limb was present in a rod monochromat, in whom cone system ERGs were indistinguishable from noise. Instead, the nonmonotonic nature of the luminance-response function may result from a luminance-dependent algebraic summation of the components that underlie the ERG waveform of the rod system.