Heterochromatic flicker photometry (HFP) was employed to estimate how the human long-wave sensitive (L) to middle-wave sensitive (Mj cone ratio varies with retinal location in five color-normal males. Measurements were made at 0 degrees, 25 degrees, and 40 degrees in the temporal retina. The HFP spectral sensitivities were fitted with a linear combination of L and M cone sensitivity functions derived from the Stockman and Sharpe(1) cone fundamentals, according to genotype. For the white (6500 K, 3 log Td) adapting field conditions, the deduced UM cone ratios of each subject were nearly constant at all measured eccentricities, with individual values ranging from 0.9:1 to 3.4:1. (C) 2000 John Wiley & Sons, Inc.