This report describes a freeze fracture study of rat dorsal sympathetic chain in normal, streptozotocin diabetic on normal diet, and a group fed a 1% myoinositol normal diet. In nonmyelinated fibres, the diabetic group had significant loss of particles on the P face of juxta-axonal Schwann cell membranes, whilst myelinated nerves showed a profound loss of particles on P and E faces. In the group with adjuvant myoinositol the particle numbers were normal in both types of nerve. Axonal plasma membranes showed increased numbers of particles in the P face of the nonmyelinated membranes in the diabetic, but were normal in the myoinositol group. Myelinated axonal membranes showed no significant difference between the diabetic and the normal. Rats on a myoinositol diet showed a significant decrease in the E face particles. These results are consistent with those described in the sciatic myelinated nerves by earlier authors and reinforce the view that myoinositol is an important membrane constituent as phosphatidylinositol and is essential for the expression of normal protein particle numbers.