The extranuclear endogenous mono-ADP-ribosylation of proteins was monitored in cellular preparations of retina, superior cervical ganglion, dorsal root ganglia and peripheral nerve. At least 6 protein fractions are ADP-ribosylated in the crude extract fraction from retina control preparations, while in diabetic rats the number of retina labeled proteins and the extent of labeling are highly reduced. In the superior cervical ganglion labeling was present in 10 proteins, in diabetics it was greatly decreased. Treatment of diabetic rats with silybin, a flavonoid mono-ADP-ribosyltransferase inhibitor, did not affect hyperglycemia, but prevented the alteration of extent of protein ADP-ribosylation. These data suggest that proteins of retina and peripheral ganglia are excessively ADP-ribosylated in vivo. The effects of silybin treatment on excessive mono-ADP-ribosylation of proteins was associated with the prevention of reduction of substance P-like immunoreactivity levels, that is typical of diabetic neuropathy. In the membrane fraction of sciatic nerve Schwann cells, at least 9 proteins were ADP-ribosylated, diabetes caused a marked increase of labeling. A comparable increase involving the same proteins is triggered by chronic nerve injury and by corticosteroid treatment. Silybin treatment of diabetic rats prevented such an increase. We propose that the inhibition of excessive protein mono-ADP-ribosylation by silybin prevented the onset of diabetic neuropathy. While the effects on Schwann cells is likely indirect and secondary to the improvement of diabetic axonopathy.