In the present study, we investigated the effects of spermine on postnatal liver maturation in suckling rats. The animals were given spermine either per os (8 μmol) or by intraperitoneal injection (1 μmol), once daily for three or five days. The percentage of liver cells in different cell cycle phases and of diploid cells in the parenchyma was estimated. The protein content, ornithine aminotransferase (OAT) activity, and content of DNA polyamines and receptors for polymeric immunoglobulins (RPI) were also measured in liver extracts. The ingestion of spermine had the following effects: the percentage of the cells in S and G2M phases of the cell cycle diminished the percentage of diploid cells increased the content of polymeric immunoglobulin receptors increased; the OAT activity increased; the contents of putrescine and spermidine decreased and almost reached adult values; and the spermidine/spermine ratio became similar to that observed in the liver of adult rats. These phenomena were detected 40 hours after the beginning of oral spermine treatment. The intraperitoneal injection of spermine had no effect on the OAT activity, but it decreased the spermidine content and enhanced the spermine content. Our data demonstrated for the first time that dietary polyamines play a role in the initiation of liver postnatal maturation in suckling rats.