1. 1. In the ventral prostates of rats undergoing normal sexual development, chromatin-associated phosphokinase activities toward dephosphophosvitin and lysine-rich histones were maximal at 48 days of age, whereas phosphorylation of endogenous chromosomal proteins was greatest in the sexually mature adult. 2. 2. Polyacrylamide gel electrophoretic analysis of chromatin proteins from these animals revealed age-dependent changes in the amount of phosphorylation of specific proteins. 3. 3. These were most evident in highly phosphorylated H2SO4-soluble nonhistone proteins and in those nonhistones (13-16 K and 20-35 K mol. wt) which are insoluble in H2SO4.