Extracellular photo-cone responses were isolated in the intact rhesus monkey eye by fractional recording across the outersegment layer in the fovea with a bipolar microelectrode. The cone response vs intensity function was determined in the presence of adapting backgrounds up to 10(6) td. Increment and decrement responses, as well as the response to the steady backgrounds were recorded. All steady state and transient response data could be described with the equation V/Vm = In/(In + sigma n) with n = 0.74 and V and I representing total response and total incident light intensity. This invariant response function is shifted both along the intensity and the response axis with increasing background intensity. The decrease in sensitivity, corresponding to these shifts, could be attributed to cellular adaptation (sigma-adaptation), pigment bleaching and response compression. An analysis of the threshold vs intensity function shows how each of these mechanisms contributes to produce Weber behaviour.