We analyzed the ratio of L:M cone photopigment mRNA in the retinas of Old World monkeys, using the method of rapid polymerase chain reaction-single-strand conformation polymorphism. The L:M cone pigment mRNA ratio in whole retina ranged from 0.6 to 7.0, with a mean of approximately 1.6 (standard deviation, +/- 0.56; n = 26). There was no change in this ratio with eccentricity up to 9 mm (approximately 45 degrees), though the ratio was approximately 30% greater in temporal than in nasal retina. The mRNA ratios are in good agreement with the L:M cone ratio in these same retinas, inferred from electrophysiological recordings of cone signal gain in horizontal cell interneurons. The correlation between mRNA ratios and physiological cone gain ratio supports the conclusion that both measures reflect the relative number of L and M cones.