Effects of the HD mutation and environmental manipulations on the open field test. Spontaneous exploratory behavior was measured in terms of the horizontal activity (A) and vertical (rearing) activity (B) in the 3 min period of the open field test at 5 months of age. Interestingly, the decrease between the first and second days was highly significant for non-enriched HD mice (double asterisk: P < 0.001) for both the number of squares crossed (A) as well as the number of rears (B). This effect was not seen in the environmentally enriched or wheel running HD mice. This suggests that the habituation of activity seen during re-testing in HD mice is attenuated through environmental enrichment or wheel running. NE: non-enriched; EE: environmentally enriched; RW: running wheel; WT: wild-type; HD: Huntington's disease; 1st: first day of testing; 2nd: second day of testing.