The oligodendroglial nuclei were purified from mouse brain with more than 97 per cent homogeneity. Cyclic AMP-independent phosphorylation of non-histone protein in oligodendroglial chromatin was studied. Morphine sulfate, in vitro, had no effect on phosphorylation. However, chronic morphine treatment resulted in an increase of phosphorylation in high molecular weight regions of sodium dodecylsulfate (SDS) electrophoresis gel. The increase was not the result of a decrease in phosphoprotein phosphatase activity.