Understanding the early factors affecting obesity development in males and females may help to prevent obesity and may lead to the discovery of more effective treatments for those already obese. The OLETF rat model of obesity is characterized by hyperphagia-induced obesity, due to a spontaneous lack of CCK₁ receptors. In the present study, we focused on the behavioral and physiological aspects of obesity development from weaning to adulthood. We examined body weight, feeding efficiency, fat pad (brown, retroperitoneal, inguinal and males' epydidimal) weight, inguinal adipocyte size and number, leptin and oxytocin levels, body mass index, waist circumference and females' estrous cycle structure. In the males, central hypothalamic gene expression was also examined. OLETF rats presented overall higher fat and leptin levels, larger adipocytes, and increased waist circumference and BMI from weaning until adulthood, compared to controls. Analysis of developmental patterns of gene expression for hypothalamic neuropeptides revealed peptide specific patterns that may underlie or be a consequence of the obesity development. Analysis of the developmental trajectories towards obesity within the OLETF strain revealed that OLETF females developed obesity in a more gradual manner than the males, presenting delayed obesity-related breakpoints, with reduced adipocyte size but larger post-weaning fat pads and increased adipocyte hyperplasia compared to the males. Intake decrease in estrus vs. proestrus was significantly less in OLETF vs. LETO females. The findings highlight the importance of using different gender-appropriate approaches to increase the efficacy of therapeutic interventions in the treatment and prevention of chronic-early-onset obesity.