Genetic influences on energy, balance and body weight were studied in four strains of rat (male Sprague-Dawley, Lister Hooded and Alderley Park, and male and female hybrid wild/Sprague-Dawley), maintained for 15 days on either a stock or cafeteria diet. Metabolizable energy intake (kiloJoule/W0.75) was similar for all stock-fed groups, and intake was increased by 34% in Alderly Park rats on the cafeteria diet, but by 50-60% in all other strains. Sprague-Dawley and Alderley Park rats were heavier and fatter than other strains, and gained 16 g more fat during cafeteria feeding compared to 8 g in the wild hybrid and only 4 g in the Lister rats. These variations in weight and fat gain resulted from differences in energetic efficiency and energy expenditure. Compared to their controls, expenditure was elevated by 63 and 21% in Lister and Alderley Park cafeteria-fed rats, respectively, and by 50-60% in other groups. Brown adipose tissue mass and protein content was increased in all cafeteria-fed groups. In the laboratory strains, resting oxygen consumption, before and after norepinephrine injections (250 micrograms/kg, s.c.), was significantly increased by the cafeteria diet. Variations in fat and weight gains among strains therefore appeared to be mainly due to differences in metabolic efficiency, and these were exaggerated by cafeteria feeding.