The influence of food-derived opioid peptides beta-casomorphines on the manifestation of nursing albino rats maternal behavior was investigated. It was shown that both acute and chronic (during the postnatal period) administration of beta-casomorphin-7 (Tyr-Pro-Phe-Pro-Gly-Pro), the typical representative of this group of peptides, decreases the level of the parental motivation. The effects of beta-casomorphin-7 were naloxone-dependent; N-terminal-reduced analogues had a significantly lesser activity. The obtained results testify in favor of the probable role of casein opioid fragments which were formed in mammary glands of a nursing female, in the development of maternal behavior abnormalities. At the same time, beta-casomorphins could be considered as the limiting factors to the excessive manifestation of the parental motivation.