AbstractFive groups of female rats which were exposed to thiouracil for varying periods around the time of birth were compared with a 6th group of untreated controls in motivational, metabolic, and hormonal test situations during adolescence and adulthood. The thiouracil‐treated rats displayed reduced fearfulness in lever‐touching and lever‐pressing tasks in operant conditioning chambers and in their initial adaptation to activity‐wheel and maze apparatuses. These rats also showed hyperactivity in asymptotic running‐wheel performance, increased spontaneous recovery of extinguished lever‐pressing, and elevated responding in lever‐pressing for variable‐interval food reinforcement. A supplemental study revealed significantly greaterad libitumfood and water intake and oxygen consumption in male thiouracil‐treated rats and elevated serum thyroxine levels in thioracil‐treated females. In general the results indicate that perinatal thyroid deficiency engenders a chronic hypermetabolic state in both sexes which may be associated with a persistent, mild hyperthyroid condition in the