Effect of training chamber on locomotor activity during repeated exposures to either cocaine or saline. Bars represent locomotor activity of groups of mice (N = 4 per strain; B6 N = 8) exposed to cocaine and saline in either the black or white chamber. The first white bar is cocaine-induced activity in white chambers. The second white bar is saline-induced activity in white chambers. Error bars are SEM. B6, C3H and FVB exhibited more locomotor activity in the black chamber regardless of treatment with either saline or cocaine. D2 mice showed no difference in activity in either black or white chambers. 129 mice showed reciprocal differences in activity and were more active in the black chamber after exposure to cocaine and more active in the white chamber after exposure to saline. BALB mice showed similar amounts of locomotor activity in response to saline in both the black and white chambers and showed a significantly higher response to cocaine only when it was administered in the white chamber.