Desiccated thyroid (Proloid) administered to healthy adult male prisoners in dosages increased progressively during a 3-month period from 3 to 25 grains per day was surprisingly innocuous as judged by the variable production of nervousness, irritability, sweating, palpitations, chest pain, increased appetite, nocturia, diarrhea or constipation. However, tachycardia, and an increase in the systolic blood pressure developed consistently in the group. During such ingestion of desiccated thyroid, in addition to tachycardia, minor electrocardiographic changes were observed. Serum PBI levels during Proloid therapy did not follow the pattern of proportionate successive increases demonstrated by others during the administration of ordinary desiccated thyroid in comparable dosages. Instead, on 3 grains of Proloid per day the serum PBI decreased below starting values and with successive increments of Proloid rose only to 7 gamma per cent. This suggests that, compared to ordinary desiccated thyroid, the preparation of Proloid produced more of a triiodothyronine effect. Such a conclusion is also supported by the marked suppression of I 131 uptake produced by the Proloid and the long interval before pretreatment PBI levels were reached following withdrawal of the medication. The serum BEI paralleled the PBI, though at a lower level; the binding of exogenous thyroxine to serum TBG and albumin as measured by electrophoresis was not affected. Proloid therapy produced an increase in triiodothyronine binding to blood cells in the group as a whole, but precise correlation in individual subjects was not evident. The contraction time of the Achilles tendon reflex as measured by the kinemometer, and contraction plus half of the relaxation time as measured by the photomotograph, indirect indices of the levels of thyroid hormones, were shortened in the groups as a whole, but again precise correlations in individual subjects were lacking. Maximal hand strength as measured by a recording ergometer was reduced during the ingestion of this preparation of desiccated thyroid on increasing dosages, indicating that a form of thyrotoxic myopathy may have been produced.