When normal mice have their usual chow diets supplemented by free access to a solution of 10% sucrose, their caloric intake increases by about 30%. We have used this model to explore the effects of sucrose overfeeding on Na,K-ATPase-mediated cation transport. After 5 days of sucrose supplementation, Na,K-ATPase mediated K uptake is increased by 88% in liver slices and 26% in intact soleus muscles of these animals. Ob/ob mice are hyperphagic on an ad libitum chow diet, and they display even greater increments in caloric intake than do thin controls when similarly allowed access to sucrose. Despite hyperphagia while on a chow diet, Na,K-ATPase-mediated K uptake by liver slices of ob/ob mice is not significantly different from that of their thin littermates. In addition, Na, K-ATPase-mediated K uptake into liver slices of ob/ob mice does not significantly increase while on sucrose supplements. These findings demonstrate the influence of dietary factors on Na, K-ATPase-mediated ion transport in liver and muscle of normal mice, and suggest that ob/ob mice may have an impairment in such dietary control. These observations suggest an important role for nutritional factors in the overall regulation of monovalent cation transport, and may also have relevance for our understanding of the cellular mechanisms of dietary thermogenesis.