Using a specific and sensitive epidermal growth factor (EGF) radioimmunoassay, the distribution of EGF in mouse kidneys was studied. In five portions (outer cortex, inner cortex, outer medulla I, II, and papilla) of kidney slices, the outer medulla revealed the highest immunoreactive EGF (iEGF) per wet weight of tissue. In microdissected nephron segments, the predominant iEGF per millimeter length was observed in the medullary and cortical thick ascending limbs of Henle's loop compared with other segments. It became more apparent when expressed per milligram protein. Fluorescence microscopy showed prominent staining for EGF in the thick ascending limbs of Henle's loop. The significance of the nephron heterogeneity in EGF contents is discussed.