To find out the mechanism of oncoprotective effects of the delta-sleep-inducing peptide, we have studied the dynamics of ribonucleotide reductase activity, which is a vital enzyme in the DNA biosynthesis, using electron paramagnetic resonance (EPR), and a concentration of low molecular mass iron complexes in the murine spleen after intraperitoneal injection of the peptide and the antitumor preparation methylnitrosourea. The data obtained give evidence on the different effects on the ribonucleotide synthesis system by the substances under study. The peptide induced significant activation of the enzyme as a consequence of activation of processes of protein synthesis. Methylnitrosourea injection leads to a decrease in the concentration of the ribonucleotide reductase active form due to destruction of the enzyme active center. The prospects of the study are discussed. © 2009 Pleiades Publishing, Inc.