The constancy of the energy expended by rats on spontaneous activity, and the distribution of activity between feeding and non-feeding.

1. Methods were developed and described for partitioning the total daily energy expenditure of rats into compartments attributable to rest, total spontaneous motor activity and feeding activity.2. In terms of energy expenditure feeding appeared as a form of motor activity which was at least as costly as other spontaneous activity.3. The compartment attributable to total spontaneous activity within a 24 hr period was about 25% of total energy expenditure and was not altered by body weight, food intake or feeding mode (e.g. solid or liquid food).4. The compartment attributable to feeding activity varied with food intake and with feeding mode in such a way that non-feeding activity varied inversely with feeding activity. By a given feeding mode non-feeding activity varied inversely with food intake.5. It is concluded (a) that food deprivation does not induce increased total activity by the rat, but does produce a complementary increase in non-feeding activity; (b) that measurement of a specific motor activity does not give a valid estimate of change in total activity; and (c) that the complementary relationship between the energy expended on feeding activity and that expended on non-feeding activity is responsible for the decline in food intake with increasing non-feeding activity that has been shown to occur at low habitual levels of spontaneous activity.