A signal-detection procedure was used to measure the influence of rods on S-cone-mediated ("tritan") chromatic discrimination in normal trichromatic human observers. Rod influence on discriminability of both increments and decrements in S-cone excitation was assessed by comparing cone-plateau (minimizes rod influence) and dark adapted (maximizes rod influence) conditions. For 1 degrees stimuli, rods impaired tritan discrimination, especially for Scone decrements. For 8 degrees stimuli, rods had no systematic effects. This confirms that rods have the same direction of effect (impairment) on tritan discrimination as has been shown for Rayleigh (L-M) discrimination However, the size dependence of the rod effect on tritan discrimination is opposite to that previously shown for either Rayleigh discrimination or hue appearance. This provides further evidence for separable rod influences on S-cone and L-M cone pathways. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S65-S68, 2001.