In monkeys, elevating ambient temperature has been shown to increase sensitivity to 3,4-methylenedioxymethamphetamine (MDMA) reinforcement. Earlier rodent studies have shown that elevations in thyroid levels (hyperthyroidism) parallel changes in elevating the ambient temperature on MDMA-induced thermogenesis, but the interaction has not been examined in monkeys. This study was designed to evaluate the effects of chronic levothyroxine (3.0 or 4.5 microg/kg/day, intramuscularly; Levo) treatment on MDMA-induced increases in body temperature following 1.5 mg/kg (intravenously) MDMA and self-administration when MDMA (0.03-0.3 mg/kg/injection) and food were available under a concurrent fixed-ratio 30 schedule of reinforcement in rhesus monkeys (n=4). Earlier studies had shown that 1.5 mg/kg MDMA did not affect thermoregulation at 24 degrees C. Chronic Levo treatment resulted in significant increases in MDMA-induced thermogenesis. In the self-administration experiment, MDMA choice increased with dose, such that food was preferred over saline and a low MDMA dose (0.03 mg/kg/injection), whereas 0.1 and 0.3 mg/kg/injection MDMA was preferred over food. Although elevating ambient temperature had been shown to increase MDMA potency, there was no effect of chronic Levo treatment on MDMA choice. These results suggest that changes in thyroxine levels do not parallel the changes in ambient temperature in altering the reinforcing strength of MDMA.