Aspirin is a widely used over-the-counter drug in our society which has wide therapeutic value, yet not all of the behavioral side effects have been studied. Different doses of aspirin solutions were administered (ip) prior to fixed-interval 2-min. schedules of microwave reinforcement in rats tested in a cold environment. Four Sprague-Dawley rats were conditioned to regulate their thermal environment with 5-sec. exposures of MW reinforcement. Friedman's nonparametric test showed significant differences among aspirin and saline-control doses. Post hoc sign tests showed that a moderate dose of aspirin increased operant behavior reinforced by MW radiation, yet lower and higher doses decreased and then increased the rate of responding which resulted in an inverted U-shaped trend. Possible multiple effects of aspirin in terms of its thermoregulatory as well as its pain-tolerance properties, and implications for hypothalamic "set point" are discussed.