Mean distance traveled scores (±SEM) of male and female rats (n = 8 per group) on the test day. Preweanling (PD 20), adolescent (PD 40), and adult rats (PD 80) were injected with saline or cocaine (2.5, 5, 10, or 20 mg/kg) immediately before testing. The inset shows data collapsed across the 12 time blocks. ‘a’ Significant difference between the saline group and the groups injected with 5, 10, and 20 mg/kg cocaine. ‘b’ Significant difference between the saline group and rats injected with 5 mg/kg cocaine. ‘c’ Significant difference between the saline group and rats injected with 10 mg/kg cocaine. ‘d’ Significant difference between the saline group and rats injected with 20 mg/kg cocaine. ‘e’ Significant difference between the 10 mg/kg cocaine group and rats injected with 20 mg/kg cocaine.