Many small rodents living in the wild neither store food nor forage during the daytime. Thus they can feed only at night. Imposing this restriction upon young female laboratory mice maintained at 22 degrees C yields a dramatic daily cycle in their fat stores. Energy is rapidly stored as fat while feeding, and then rapidly utilized during the non-feeding period. Almost one-third of the extractable whole body fat is lost during a 14 hour non-feeding period. Less fat is stored while feeding at 11 degrees C. Thus missing a single feeding period at this cooler temperature results in a total depletion of fat stores. In an ultimate sense then, the daily challenge of surviving with such a paucity of fat reserves probably presents as great a problem to the small mammal as does the thermoregulatory cost of small body size itself. Strategies for solving this problem apparently vary immensely from population to population and from locale to locale.