Thyroid function was studied in unanesthetized female rats by determining 125I levels in sequential blood samples obtained via chronic jugular catheters. On the basis of plasma corticosterone determinations, nothing associated with the sampling procedure was stressful for the animals. Synthetic TRF and TSH injections resulted in rather rapid increases in blood 125I levels. Exposure to cold (either continuous or transient) caused a similar but somewhat delayed response. The response to cold was abolished by thyroxine pretreatment. Pentobarbital anesthesia, when maintained by hourly injections, also abolished the cold response. This was, at least in part, due to decreased responsiveness of the thyroid to TSH.