Sixteen male hamsters, maintained on a 12-hour light-dark cycle, were observed in an open field at six predetermined times in their activity cycle. The experimental group (N = 6) was injected intraperitoneally with 1.5 g/kg ethanol (10% w/v) immediately prior to observations. Control animals were either injected intraperitoneally with a physiological saline solution (N = 5) or were without treatment (N = 5). Open field measures were the number of squares entered during a five-minute period (locomotion) and latency (in seconds) to reach the wall from the center. It was found that ethanol affected the animals differently during light and dark phases. Locomotion measures indicated an increase in activity during dark phases and a decrease during light phases, relative to the baseline. An increase in latency was found during the light phase for experimental animals relative to baseline measures. A group x time of administration interaction for locomotion was also found. These findings suggest that factors such as time of administration may influence the effects of ethanol and that keeping time of administration constant is important when investigating drug effects.