The growth of the rat calf muscle has been followed radiographically during neonatal undernutrition and subsequent catch-up. The following observations were made: (1) The mean calf muscle width of rats reared in litters of 16 was 20% less than that of those reared in litters of 8. (2) There was no significant difference between the sexes in the degree of growth retardation brought about by suckling the pups in large litters. (3) By the end of the experiment, females had caught up in both muscle width and muscle length, the males had caught up in muscle length (identical to tibia length) but not in muscle width. (4) In male pups during undernutrition the muscle diameter was greater than in normally fed animals of the same weight (but younger). This difference was not seen in females. (5) During recovery, until a weight of about 160 g was reached, the muscle width of the neonatally undernourished rats of both sexes was less than controls of the same body weight.