This paper provides a procedure for estimating the maximal stationary metabolic rate (SMRmax) in free-moving rats. The SMRmax was estimated by simultaneously comparing energy expenditure with motor activity measured at 10 min intervals for 24 h. The SMRmax was close to but naturally higher than the resting metabolic rates determined by other conventional methods. The coefficient of variation for 3 con-secutive days was below 3%. The ratio of SMRmax to daily minimum energy expenditure was 1.25 in young rats and 1.19 in adult rats. SMRmax is a useful key parameter for analyzing daily energy expenditure and behavior.