A single-subject design was used in 2 experiments about the effects of percentage payback (winnings in proportion to total amount bet) on gambling on a slot-machine simulation in 8 adult humans. In Experiment 1, percentage payback was varied across a wide range of values, and participants were exposed extensively to percentage-payback conditions. Gambling did not vary systematically across levels. In Experiment 2, win probability and win size were manipulated across conditions while percentage payback was held constant. One participant gambled more in the high-win-probability, low-win-size condition than in other conditions; however, gambling by the remaining 2 participants was similar across conditions. Most participants placed fewer bets as each experiment progressed, suggesting that behavior was sensitive to the monetary loss typically produced by gambling. Overall, the results highlight the utility of procedures that provide repeated and extensive experience with gambling contingencies. (Contains 7 figures.)