Normal and hyperthyroid rat pups received intraoral infusions of 0.25% quinine hydrochloride at 8, 10, 12, 14, or 16 days of age. Chin scraping and paw treading, aversive behaviors that adult rats show to quinine, emerged at 12 to 14 days in normal animals and were accelerated in hyperthyroid animals. Once these behaviors emerged, the hyperthyroid pups showed levels of both paw treading and chin scraping that were typical of controls several days older. Hyperthyroid and control pups did not differ in their intake of quinine at ages at which there were marked differences in chin scraping and paw treading. Other types of responses that pups show to aversive stimuli, such as paw treading. Other types of responses were not affected by the thyroid treatment. These findings suggest that the appearance of chin scraping and paw treading at 12-14 days of age in normal animals results from maturation of motor systems underlying these behaviors, and that this maturation can be accelerated by treatment with thyroid hormones.