A study of body weight of male and female albino rats from the time of initiation of a thyroid or parathyroid deficiency at 23, 30, 50, 65, 75 or 100 days up to the time when the animals were 150 days old, brought out the following: (1) The role of the thyroid in growth is that of a participant in the regulation of the metabolic level through which is determined the rate of the maintenance processes and hence the amount of material presented to the cells for growth during a given period. It is therefore a determinant of the capacity factor in growth. (2) Catalysts other than those produced by the thyroid are concerned in determining the intensity factor in growth. (3) The thyroid is more largely concerned in growth by increase in cell mass than in growth by increase in cell number. (4) The parathyroids are not concerned in growth except in so far as they protect the organism from the growth-retarding influences exerted by toxic products resulting from parathyroid deficiency. The resulting retardation of growth is attributable largely to a lowering of the nutrient level following a disturbance in effectiveness of the digestive system due to the response of the sympathetic system to the toxemia. (5) Puberty affects the growth response to glandular deficiencies. This influence originates in gonadal incretory development and is expressed as an increased need for growth materials. (6) The fact of most significant biological interest is the demonstration of the stability of the intensity factor of growth under adverse conditions and the remarkable power of the organism to adjust itself in this respect.