Conformational changes in the chromatin of the cerebral hemisphere of 3-, 14- and 30-day old developing rats were studied before and after its ADP-ribosylation using DNase I and micrococcal nuclease (MNase). The rate and extent of digestion of chromatin by DNase I are the highest at 3-day and decline progressively thereafter. The rate and extent of digestion by MNase do not change during development. ADP-ribosylation of chromosomal proteins was carried out by incubating nuclei with NAD+ for 30 min and was followed by endonuclease digestion. Both the rate and extent of digestion by DNase I and MNase were enhanced after ADP-ribosylation which was the maximum for 3-day rats.