Twenty-eight-to 33-and 120-to 140-day-old male Sprague-Dawley rats were injected with physiological saline, 2.0, or 5.0 mg/kg d-amphetamine. Thirty minutes after the injection, they explored an open field for 8 min and were then trained in a one-way avoidance task. Mature animals were more active in the inner portion of the open field following 2.0 mg/kg and acquired the avoidance task more rapidly than those given saline or 5.0 mg/kg. Neither drug dose altered activity or avoidance of immature animals. Both mature and immature rats had shorter escape latencies to footshock following the drug. Differential effects of d-amphetamine in mature and immature animals are probably due to developmental changes in brain norepinephrine and/or dopamine.Very few studies have compared the behavioral effects of drugs in mature and immature animals, in spite of the fact that levels of some central nervous system chemicals altered by drugs differ in mature and immature animals. In one of the few reports comparing different age groups treated with d-amphetamine (d-A), Sofia (1969) found that 1.0 mg/kg increased activity of 32-day-old rats, but that this increase was less than in mature rats. Although no statistics were reported, Campbell, Lytle, and Fibiger (I 969) concluded that doses of .25-8 .0 mg/kg d-A produced similar activity increases in 10-, 15-,20-, 25-, and lOO-day-old rats.