Oligodendroglial nuclei isolated from rat brains at different stages of myelinogenesis (10, 18, and 30 days of age) were incubated with [gamma-32P]ATP and extracted with 0.75 M perchloric acid to yield a fraction of nonacidic chromatin proteins. The protein extracts were then analyzed by polyacrylamide gel electrophoresis. The phosphorylation pattern of these proteins was found to be different for different age groups. In 10-day-old rat oligodendrocytes the most extensive phosphorylation occurred in low molecular mass species (less than 30 kDa), in contrast to fractions obtained from 18- and 30-day-old rat oligodendrocytes which showed a significantly higher labeling of the proteins with molecular masses greater than 30 kDa. The phosphorylation of the latter species was greatly stimulated by the presence of cyclic AMP in the incubation media. The results suggest that the phosphorylation of specific nuclear proteins, which may play a regulatory role at different stages of oligodendroglial maturation and myelinogenesis, may be at least partially modulated by intracellular cyclic AMP.