Three techniques for overfeeding rat pups were used from birth to 24 days of age to assess their effect on growth and development in an obesity susceptible (OM) and an obesity resistant (S 5B/P1) strain of rat. These included: (1) Feeding a semi-purified high fat rather than low fat diet to the dams (alters the energy content of milk); (2) Raising pups in litters of 3 rather than 6; and (3) Force feeding of milk in lieu of any force feeding. In OM rats there was no difference in lean body mass (excludes heart, liver and kidney) (LBM) gastrocnemius muscle weights, cell number and protein content of muscle. Body weights and gastrocnemius muscle triglycerides were significantly higher in OM rats fed the high fat diet than in those fed the low fat diet. S 5B/P1 (S) rats treated in the same manner as OM, showed no significant differences in body weight, LBM, gastrocnemius muscle weight or content of DNA, protein or triglycerides. Protein and DNA for 100 g fresh gastrocnemius muscle were similar for the two strains even though the muscle was 1 1/2 times heavier in OM than in S rats. Muscle triglycerides were higher in OM than in S rats.