The purpose of the current investigation was to characterize further the taste discriminative stimulus properties of quinine. Using a food-maintained two-choice (quinine and water) discrimination paradigm, the effects of varying inter-trial intervals, and the number and distribution of test stimuli were examined in rhesus monkeys(Macaca mulatta).Monkeys were trained to respond under a chain schedule of reinforcement (FR5B-fluid, FR30B-food) with a 60-s inter-trial interval. Interestingly, the quinine discrimination remained invariant across all the conditions tested: alterations in inter-trial interval (0, 15, 30, 45, 60s), alterations in the number of test stimuli (six at quarter-log unit increments [0.1–1 mg/ml] versus 10 at quarter-log unit increments [0.01–1 mg/ml]), and alterations in the distribution of the test stimuli (quarter-log unit increments [0.1–0.1 mg/ml]) versus half-log unit increments [0.01–1 mg/ml]) failed to alter the quinine discriminative stimulus (ED50s ranged from 0.18–0.29 mg/ml). Furthermore, incorrect lever selections were very rare (less than 12% of all test sessions), independent of test stimuli order (i.e. quinine followed by water trials, water followed by quinine trials), and independent of the progress through the test session (i.e. incorrect lever selections were consistent - approximately one error for every 10 trials - within early, middle and late portions of the session). These results demonstrate the reliability and robustness of the quinine taste discriminative stimulus in rhesus monkeys and provide further validation for the experimental paradigm.