The dopamine biosynthetic machinery of intact synaptosomes of rat striatum showed a 5-fold increase in development from 3-day-old neonates to adults, and it was fully developed between 2-3 weeks after birth. Concurring with this development was the appearance 2 weeks after birth of a regulatory mechanism(s) through which amphetamine in vivo induced an inhibition of dopamine biosynthesis. The inhibition was not appreciably reversed when haloperidol, in addition to amphetamine, was administered.