Latent obese-hyperglycemic mice exhibit significantly subnormal heart rate and level of oxygen consumption as early as three and eight days post-partum, respectively. Also, loss of body weight during a three-hour fasting period is detectably less extreme in obese than in normal litter mates as early as five days post-partum. Regression analysis of the differences between the latent obese and normal litter mates suggests that these symptoms begin developing at birth.