Peripheral nerves in rats fed a diet containing 40% galactose develop endoneurial oedema related to the accumulation of interstitial galactitol and sodium. Biochemical studies of the perineurium in galactose neuropathy indicate an increase in membrane bound Na+/K(+)-ATPase activity. The present study has analysed the intramembraneous particles (IMPs) on the P membrane fracture faces of the perineurium in galactose-fed rats and has demonstrated a significant increase in the density of IMPs. This finding may reflect changes in the perineurial cell membranes in galactose neuropathy, such as increased membrane bound Na+/K(+)-ATPase activity.