Decision theorists typically emphasize two decision variables, the probability and magnitude of reward. Empirical research suggests that the basal ganglia (BG) are important for encoding or learning about these variables. To determine how choice behavior in monkeys varied according to these variables, we trained a monkey in a free-choice task based on Herrnstein's Matching Law. Offset of a central target cued the monkey to choose by making a saccadic eye movement to one of two peripheral targets for which rewards were scheduled at different independent rates. If a reward had been scheduled for a target the monkey chose, fluid reward was given. If a reward had been scheduled for a target not chosen, the reward remained available (Sugrue et al. SfN 2001). In a daily session we varied the probability or magnitude of the rewards associated with each target over 3-8 blocks of 100-250 trials. The log of the ratio of choices was linearly related to the log of the ratio of average rewards with slopes typically les...