The blot-hybridization technique-assisted we have studied the pattern of fragmentation by mirococcal nuclease (MNase) of DNA tyr-aminotransferase (tat) and trp-dioxygenase (to) genes in active (in rat cell liver nuclei) and repressed (in brain nuclei) states. It was provided, over a wide range of enzyme concentration two types of fragments are mainly produced: near full-size to- and tat-transcription unit (19,000 and 11,000 bp, respectively) and their large (from 1500 bp) heterogeneous in length. To-and tat-fragments of both kinds are preserved in hydrolyzates at limit of MNase digestion of total chromatin DNA when nuclease breaks occur in nearly all accessible sites of chromatin. This means that these fragments originate from two distinct subsets of transcription units coexistent in liver nuclei. The first of them do not contain MNase accessible sites over their entire length, whereas in other resistant regions alternate with rare irregular located MNase-sensitive segments. We presume that resistance to MNase within transcription units is peculiar to the competent genes. As a result of transcription MNase-sensitive areas arise which possibly flank elongating RNA polymerases.