We studied expression of the immediate early gene c-fos in hypothalamic paraventricular nuclei in rats with different prognostic resistance to emotional stress receiving delta-sleep-inducing peptide after intracerebroventricular administration of cycloheximide. Delta-sleep-inducing peptide inhibited expression of the c-fos gene in rats receiving intracerebroventricular injection of physiological saline. Pretreatment with the protein synthesis blocker cycloheximide abolished changes produced by delta-sleep-inducing peptide.