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Intracerebroventricular administration of the peptides kyotorphin (Tyr-Arg), neokyotorphin (Thr-Ser-Lys-Tyr-Arg), and Asp-Tyr at doses of 4 and 8 μg altered the behavior of rats in a manner similar to that seen after similar administration of brain fractions from hibernating ground squirrels ( Citellus undulatus ), which contained these peptides: there were increases in orientational reactions, increases in the frequency of stereotypical scratching movements, grooming, yawning, hiccuping, and sneezing. Animals became drowsy after 15-20 min. Peptides and brain fractions also had similar effects on the EEG of rats. Brain fractions reduced theta and alpha rhythms and enhanced delta and beta frequencies. Increases in delta waves were seen with all peptides (a 4-μg dose of kyotorphin produced alternating increases and reductions in the delta rhythm). Inhibition of theta and alpha rhythms after administration of Asp-Tyr and kyotorphin was more transient than after brain fractions. Increases in beta frequencies were seen only after administration of 8 μg of Asp-Tyr, the smaller dose not producing this effect.