To determine whether dietary iodine intake affects the levels of immunoreactive epidermal growth factor (EGF) in vivo in two mouse tissues known to be thyroid hormone responsive, 24 adult BALB/c mice were randomized into groups of six animals and fed a) an iodine-deficient diet, b) normal laboratory chow (controls), c) an iodine-deficient diet with high physiological iodine supplementation (approximately 300 micrograms/day), and d) an iodine-deficient diet with pharmacological doses of iodine (approximately 3 mg/day). After 21 days on the various regimens, the mice were killed and the concentration of mouse (m) EGF in the thyroid and submaxillary glands (SMG) measured by radioimmunoassay. The mean (+/- SEM) SMG mEGF level (micrograms/mg wet weight) in the different groups were: a) 7.58 +/- 1.87, b) 12.46 +/- 4.7, c) 19.79 +/- 4.47, and d) 24.36 +/- 5.96. The thyroid mEGF concentrations (ng/mg wet weight) were: a) 3.65 +/- 0.75, b) 5.23 +/- 1.84, c) 8.07 +/- 1.18, and d) 6.43 +/- 0.95, respectively. Analysis of variance revealed that dietary iodine status had a significant effect on mEGF levels in both the SMG (p < 0.001) and the thyroid gland (p < 0.001). Compared to controls, tissue mEGF levels decreased by 30-40% in the mice fed an iodine-deficient diet, and increased by 60-150% following physiologic iodine replacement.