The adult mouse brain contains complex populations of polyadenylated [poly(A)+] and nonpolyadenylated [poly(A)-] messenger RNA's (mRNA's). These mRNA's are separate sequence populations, similar in complexity, and in combination are equivalent to approximately 150,000 different mRNA sequences, of average length. Essentially all of the "adult" poly(A)+ mRNA's are present in the brain at birth. In contrast, most of the poly(A)- mRNA's are absent. Brain poly(A)- mRNA's begin to appear soon after birth, but the full adult complement is not reached until young adulthood. This suggests that these poly(A)- mRNA's specify proteins required for the biological capabilities of the brain that emerge during the course of postnatal development.