The levels of d‐ and 1‐amphetamine in the brain and heart of adult and developing mice aged 3–6 days and 13–16 days were studied. More d‐ than 1‐isomer was found in the brain of developing mice, except after a dose of 100 mg/kg of the drug, i. e. a dose which produces a higher level of 1‐amphetamine in the brain of 13–16‐day‐old mice. Adult brain retained 1‐amphetamine in a higher concentration than d‐amphetamine. In the youngest mice 50 mg/kg of the isomers produced equal levels of the drug in the brain and heart, whereas 100 mg/kg of the d‐form was particularly concentrated in the infant heart. In the older age groups the brain level of either isomer was generally higher than the respective heart level. The age‐ and dose‐dependent differences in the distribution of amphetamine isomers emphasize the importance of peripheral component of amphetamine action in the youngest mice and reveal a low sensitivity of the central nervous system to 1‐amphetamine in 13–16‐day‐old as well as in adult mice.