Male rats were undernourished for one of three periods of time during suckling. They were subjected to undernutrition from birth to 8 days post-partum, birth to 15 days or birth to 22 days. The growth of the humerus, radius, femur and tibia was followed radiographically during the undernutrition and recovery periods. It was found that the lengths of these bones in animals undernourished from birth to 8 days were able to recover completely, while after the two longer periods of undernutrition, the animals were unable to recover. The results are not in complete agreement with those of previous workers and it is suggested that this may be due to differing rates of growth and maturation between the animals used in different studies.