The concentrations of epidermal growth factor (EGF) were measured by radioimmunoassay in the submandibular gland, plasma and urine of adult female C3H/HeN mice whilst virgin and during pregnancy, lactation and after lactation. During gestation there was a significant increase in the submandibular EGF concentration which was five to seven times higher than that found in virgin mice. The level of EGF in the gland remained high during the period of lactation and even several weeks after lactation. Plasma EGF levels were also increased during the periods of pregnancy, lactation and after lactation when compared with those of virgin mice. These increases were, however, apparent only between 24.00 and 08.00 h, because of circadian variations in circulating EGF. The level of plasma EGF was significantly (P less than 0.05) higher during the 24.00-08.00 h period than during the 12.00-20.00 h period in all stages examined. Concentrations of EGF in the urine of virgin, pregnant, lactating and primiparous mice remained relatively constant, and the levels were much higher than those in the plasma. Similar studies using sialoadenectomized pregnant and lactating mice indicated that the plasma levels of EGF were below the level of sensitivity of the assay (less than 16.5 pmol/l (less than 0.1 ng/ml] even during the 24.00-08.00 h period, whereas urinary EGF remained at high levels which were similar to those of normal pregnant and lactating mice. These results suggest that submandibular EGF contributes to the increase in plasma EGF which occurs after gestation, but is not the major source of the urinary EGF.