We have investigated the physiological factors which regulate transferrin gene expression in the mammary gland of the rat. Our studies by dot blot analysis have demonstrated that multiple doses of 17β-oestradiol (OE2; 0·5 mg/kg per day for 3 days) elicit a specific 3·5-fold increase in the transferrin mRNA levels in the mammary glands of virgin rats. The hormonal action of OE2 in mammary tissue was specific for the transferrin gene, as judged by hybridization with β-actin cDNA. The accmulation of transferrin mRNA induced by OE2 treatment was similar to the developmentally regulated expression of the gene observed during the reproductive cycle. The steady-state level of mammary transferrin mRNA increased by up to 4·5-fold at day 21 of lactation, when compared with virgin and pregnant rats. Our results show that the pattern of transferrin gene expression is different in mouse and rat mammary glands. The specific response of the transferrin gene to OE2 was not found in the liver or in the uterus. In the uterus alone, OE2 produced a significant increase in the content of nucleic acids and also induced the accumulation of transferrin and β-actin mRNAs. We have detected for the first time an induction of transferrin gene expression in the mammary gland in response to OE2, and these results support the view that the pattern of transferrin gene multimodulated expression is tissue- and species-specific.