Thyroxine secretion rate has been estimated in the rat using the radioactive thyroxine turnover technique. The influences of age and sex have been examined. The thyroxine turnover technique gives values which are comparable with those obtained with methods using thyroxine replacement and restoration of basal metabolic rate, pituitary morphology, blood thyrotropin concentration in thyroidectomized rats, and with measurements based on intrathyroidal thyroxine turnover. In contrast with older methods, the turnover technique measures only thyroxine degradation, not the thyroxine equivalent of the biological effectiveness of secreted thyroxine and triiodothyronine, heretofore referred to in the literature as the “thyroxine secretion rate.” The fractional turnover rate is greater in female than in male rats, both in the Osborne- Mendel strain of age 2–3 months and the Wistar strain of 12 months. Thyroxine degradation per unit body weight is greater in females than males only in the latter group. The distribution space expressed per unit body weight is not significantly different between the sexes. Small but statistically significant age and strain differences in plasma hormone concentration (protein-bound iodine) were seen. Exposure to cold accelerates fractional turnover rate in both sexes to about the same extent. In males the distribution space is also appreciably increased, but in females the small increase is not statistically significant. Thyroxine degradation is significantly greater in male than female rats exposed to cold. There is no apparent effect of age on fractional turnover, similar values being seen in 3-, 12- and 24-month-old female Wistar rats. In all but the senescent animals, both thyroxine distribution space and degradation are closely correlated with body weight in an apparently linear fashion. The distribution space per unit body weight is significantly greater in the 24- month than the 3- or 12-month-old animals, despite the fact that the 12- and 24-month-old animals are of approximately the same weight. Thyroxine degradation is increased by 50% in the senescent rat. This unexpected finding is in contrast with results obtained in man, and with the previous estimates of thyroid function in the senescent rat.