Aspartame (L-aspartyl-L-phenylalanine methyl ester) was administered intraperitoneally to 9 Sprague-Dawley rats partitioned into 2 studies (4 in Study 1 and 5 for a replication in Study 2) over a two-year period using a within-subjects, repeated-measures reversal design. Behavioral thermoregulation was assessed in a cold Skinner Box using 5-sec. exposures of microwave radiation [Specific Absorption Rate = 0.34 Watts/kg/(mW/cm2)] as reinforcing stimuli under a fixed-interval 2-min. schedule of positive reinforcement. Two factorial analyses of variance [5 (doses) x 8 (hours)] indicated that the main effect for the doses of aspartame (2, 4, 8, 16 mg/kg, and saline control) was not significant; yet, the interaction (dose x hours) was significant (p < .05). Tentatively, aspartame should not cause an uncomfortable rise in body temperature (as sugar can do) when consumed in common substances such as soft drinks, yogurt, tea, coffee, etc., in doses commensurate with "hedonic" sweetness.