The aim of the present study was to investigate the short and long term effects of a high fat Western Diet (WD) on intake, storage, expenditure and faecal loss of energy as well as effects on locomotor activity and thermogenesis. WD for only 24 hours resulted in a marked physiological shift in energy homeostasis, including increased body weight gain, body fat and energy expenditure but an acutely lowered locomotor activity. The acute reduction in locomotor activity was observed after only 3-5 hours on WD. The energy intake and energy absorption was increased during the first 24 hours, lower after 72 hours and normalized between 7 and 14 days on WD compared to mice given chow diet. Core body temperature and energy expenditure (EE) was increased between 48-72 hours, but normalized after 21 days on WD. These changes paralleled plasma T3 levels and UCP-1 expression in brown adipose tissue. After 21 days of WD, energy intake and absorption, EE and body temperature was normalized. In contrast, the locomotor activity was reduced and body weight gain was increased over the entire 21-day study period on WD. Calculations based on the correlation between locomotor activity and EE in 2h intervals at day 21-23 indicated that a large portion of the higher body weight gain in the WD group could be attributed to the reduced locomotor activity. In summary, an acute and persisting decrease in locomotor activity is most important for the effect of WD on body weight gain and obesity in mice.