Epidermal growth factor (EGF) is one of the major growth-promoting agents in milk. Using immunohistochemistry we localized EGF in the mammary glands of lactating rats to the luminal border of the secretory cells. Following proteolytic pretreatment of the histological sections, the EGF-immunoreactivity was revealed homogeneously in the cytoplasm of the secretory cells, which might suggest that EGF is present as a precursor molecule in the mammary glands. Altered glucose metabolism during lactation results in secondary hypoinsulinaemia in the lactating rat. As insulin is also known to affect lactation in several species, we treated normal lactating rats daily with insulin and studied the effect on the composition of milk. A significant increase in the content of total protein and milk fat was observed after a few days of insulin-treatment, as compared to a control group [total protein: 50 (36-97) g/l vs. 42 (35-72) g/l], [milk fat: 35 (22-40)% vs. 29 (23-36)%], [median (range)]. On day 16 the EGF concentration in milk was significantly increased in insulin-treated rats, as compared to controls [2.66 (1.40-5.08) nM vs. 1.98 (1.04-3.16) nM]. A similar significant increase was found for the secretion of the cobalamin-binding protein, haptocorrin (HC) [37.7 (15.8-110.4) nM vs. 23.5 (15.5-70.1) nM]. In conclusion, the highly insulin-sensitive lactating mammary glands were affected by exogenous insulin, since the milk concentrations of EGF, HC, total protein and the fat percentage were increased.