Experimental diabetic neuropathy includes the nonenzymatic glycosylation (or glycation) of the major proteins of peripheral nerve myelin. We have used X-ray diffraction to determine whether such glycation af'fects myelin membrane structure and interactions in peripheral nerves from experimental diabetic rats. Streptozocin at 60 mglkg was injected intraperitoneally to induce diabetes; controls were pair-fed and age-matched. Animals were sacrificed periodically from 2 weeks to after 1 year. The dissected sciatic nerves were tied off and incubated overnight at room temperature in hypotonic saline of defined pH and ionic strength or in distilled water. Such treatments have been shown to result in systematic changes of myelin period, which can be detected using X-ray diffraction, and which may indicate alterations in inter-membrane interactions owing to changes in composition. We observed no differences in repeat periods between control and diabetic nerves at pH 4.0 and 7.4, and ionic strength 0.01, 0.02, 0.06, 0.15, and 0.18; however, we did detect a significant difference (P < .02) in their maximum extent of swelling in distilled water: control nerves showed a period of 292 A (s.d. 23 A; n = 12) compared to 272 (s.d. 19 A; n = 11) for diabetic nerves. To determine whether this difference in swelling was due to an alteration in the properties of the apposed, extracellular surfaces of the myelin membranes or to the connective tissue in peripheral nerve, we compared the X-ray patterns from peripheral nerve myelin isolated by sucrose density gradient centrifugation from sciatic nerves of diabetic and control rats. No difference in the patterns was observed. Since the maximum extent of myelin membrane swelling in peripheral nerves is limited by the collagen, our results suggest that the collagen in the nerves of diabetic rats is less elastic or more densely packed than in the controls, most likely due to N-glycation-mediated cross-linking.