Male rats aged 6 weeks, 6 months, and 18 months were kept at 4 to 5 degrees C for 12 days and compared with age-matched controls at room temperature as to survival, weight change, food consumption, core body temperature, and several parameters of thyroid hormone economy. After 12 days in the cold, 14%, 50%, and 71% of 6-week-, 6-month-, and 18-month-old rats, respectively, were living. Weight declined in both older groups, though at a faster rate in the oldest. Increasing age did not affect serum T3, rT3, TSH, or the ability to convert T4 to T3 by 20% liver homogenate. Cold exposure, however, resulted in an increase in serum T3 and a decrease in T4 to T3 conversion by liver homogenate in a similar magnitude both in mature and early senescent rats.