Effect of oral MPD on locomotor activity on PND 23. A. Means (±SEM) of total distance traveled are shown for the treatment groups during the baseline period and during 10-min time blocks (TB) after administration of MPD. From 10 min to 40 min after dosing, that is, time blocks 2 through 4, the MPD-treated group traveled more distance than did the vehicle-treated group, with a peak of activity approximately 30 min (TB3) after dosing. * Significant difference between the vehicle group and the MPD-treated group (p<0.05). B. Regression lines for the relationship between baseline activities and the square-root of individual total distance traveled are shown for the vehicle and the treated groups. Time blocks 2, 3 and 4 are collapsed for illustration. From 10 min to 40 min after dosing, MPD increased distance traveled in animals more active at baseline but not in animals with a low level of baseline activity. Slopes of the two regression lines were significantly different (p≤0.001).