The solution conformation of rat epidermal growth factor (EGF) has been investigated by proton n.m.r. techniques. Two-dimensional proton n.m.r. experiments have allowed sequential resonance assignments to be made for most protons. On the basis of these assignments, two regions of anti-parallel β-sheet structure have been derived from the n.m.r. data. A β-sheet segment running from about V19 to V23 (capital letters refer to amino acids in the single-letter notation) is folded onto a β-sheet segment running from R28 to N32 and joined by a chain reversal from E24 to D27. A second region involves a β-turn from V34 to Y37, which starts a short β-sheet up to G39, followed by a chain reversal up to Q43, which leads to folding of the C-terminal β-sheet segment, i.e. H44-R45, running antiparallel to the short Y37 β-sheet segment. The N-terminal segment up to G18 exists in a multiple bend conformation and is folded onto the V29-V23/R28-N32 β-sheet such that Y10, Y13, Y22 and Y29 are proximal to each other. Structural comparison of rat, murine and human EGFs indicates a number of highly conserved structural features common to at least these species of EGF.