One of the characteristics of growth is the retention within the organism of what Robertson referred to as the "substrates" of growth, and of these the two subjected to the most frequent analysis have been nitrogen and calcium. As a measure of growth of active protoplasmic tissue and of the bony skeleton, weight is notoriously unreliable, and height is significant only over long periods. We have previously shown prolonged gain in weight with a negative nitrogen balance and retention of this substance for comparatively long periods with a failure to gain. Accordingly, in attempting to evaluate the effect of thyroid on growth we have used the balance method for these two substances under a variety of conditions and with variations in dosage as criteria. While originally it was planned to present both sets of material together, it has become evident that the behavior of the one substance does not necessarily