In this work, we evaluated the effect of acetyl-L-carnitine supplementation on the presence of NADPH-diaphorase positive myenteric neurons in the distal colon of rats with diabetes mellitus induced by streptozotocin. Rats 105 days old were divided into four groups: normoglycemic, normoglycemic supplemented with acetyl-L-carnitine, diabetic and diabetic supplemented with acetyl-L-carnitine. Diabetes was induced by the administration of streptozotocin (35 mg/kg, i.v.). Supplementation with acetyl-L-carnitine was done for 105 days. The neuronal density was similar in all groups. In diabetic rats, the area of the neuronal cell body profile was greater (p<0.05) than in normoglycemic rats, whereas in diabetic rats receiving acetyl-L-carnitine the areas were smaller than in the non-supplemented diabetic rats (p<0.05). The increase in the colonic area of diabetic rats was greater than in diabetic rats treated with acetyl-Lcarnitine (p<0.05), indicating that the increment in the population of these neurons was higher in treated diabetic rats. These results suggest that the beneficial effect of carnitine is restricted to preventing an excessive increase in neuronal area. The greater dilatation of the distal colon seen in diabetic rats supplemented with acetyl-L-carnitine probably represents an adverse effect of this compound.