The OLETF rat model of obesity has been extensively studied as an adult model of hyperphagia-induced obesity. In order to better understand the early circumstances that make OLETF pups obese, we investigated body weight from postnatal day (PND) 1 and examined diurnal maternal behavior over the first three postpartum weeks by undisturbed observations. Male and female OLETF rats weighed significantly more than LETO controls from PND 1 until adulthood. Differences in nursing behavior were observed only in the third postnatal week: OLETF dams exhibited more frequent nursing. OLETF dams displayed less self-directed behaviors and activity, over the entire postpartum period examined. Taken together, these data demonstrate that OLETF males and females are heavier from birth and suggest that maternal factors may contribute to weight differences between the strains.