SummaryC57BL/6J obese (ob/ob) and lean mice fed ad libitum on a normal mouse chow diet (Normal), were compared with lean mice of the same age and strain fed ad libitum on a high-fat diet, consisting of the Normal diet with the addition of beef lard (Lard), from age 3 months for 34 days. The lard-fed mice were seen to have significantly higher (P < < 0.05) body weight in this 34-day period than that of the other two groups fed on the Normal diet. Epididymal fat depot and adipocyte cell size were signi® cantly larger (P < < 0.05) in the Lard-fed lean mice and in the obese (ob/ob) mice than were those of the Normal-fed lean mice. Dietary Lard intake did not signi® cantly affect concentrations of plasma triglyceride although those of plasma cholesterol were signi® cantly increased (P < < 0.05). The development of obesity in these Lard-fed mice appeared to be accelerated and signi® cant. IUBMB Life, 48: 109± 113, 1999Keywords Diet-induced obesity; genetic obesity; high-saturated fat diet; mice. INTRODUCTIONExcess fat mass, characterising obesity, usually results from excessive energy storage over a prolonged period of time. This energy imbalance can theoretically result from excessive energy intake and lower energy expenditure, such as abnormal metabolic processes, impaired thermoregulation, or reduced physical activity. The development of obesity is believed to be in¯uenced by a number of factors, including genes and environment. Studies in animal models have clearly demonstrated that these 2 distinct types of obesity exist and that their mechanisms of excess fat mass accumulation may differ substantially (1). First is genetic obesity, as seen in rodent strains such as the Zucker fatty (fa/fa) rat and the obese (ob/ob) mouse used in this