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Kidneys removed from rats 6 weeks after thyroidectomy have been found to have low transamidinase activities which were similar to those found in kidneys from hypophysectomized rats. Injections of growth hormone into thyroidectomized rats or of thyroxine into hypophysectomized rats were without any effect on the low enzyme activities, whereas the injections of growth hormone into hypophysectomized rats or of thyroxine into thyroidectomized rats did have an effect on the low transamidinase activities. Thyroxine is concluded to have a concomitant or prerequisite role in the direct regulation of transamidinase activities by growth hormone. A bioassay for thyroxine based on the transamidinase activities of kidneys from thyroidectomized rats has been developed. (Endocrinology 87: 1237(Endocrinology 87: , 1970 K IDNEYS removed from rats one week after hypophysectomy have only a fraction of the transamidinase activities found in kidneys from sham-hypophysectomized rats (1). Hypophysectomized rats that had received injections of bovine growth hormone had greater kidney transamidinase activities than hypophysectomized rats, and the activities were an increasing linear function of the log of the dose of the growth hormone injected (1). This is to report that kidneys removed from rats six weeks after thyroidectomy have only a fraction of the transamidinase activities found in kidneys from shamthyroidectomized rats. Thyroidectomized rats that had received injections of thyroxine had greater transamidinase activities than thyroidectomized rats and the activities were an increasing linear function of the log of the dose of the thyroxine injected. Details of a bioassay for thyroxine and studies on the nature of the apparent reg-