The histological changes in the inner ear were compared with the hearing ability in insulin deficiency diabetes of rats. 38 LEW-Han-rats were treated with streptocotocin; 34 of these became diabetic, as proved by the glucose tolerance test. 22 rats served as controls. The diabetes was not treated. For a period of up to 440 days the weight, blood glucose and auditory function were controlled in the living animals. Auditory function was tested by means of pinnal reflex of Preyer for a frequency range between 1000 and 20000 cps. With regard to the mean values of frequency, neither a decrease nor a difference between normal controls and diabetic rats was found to any substantial degree. Histological examinations of sacrificed diabetic rats meanwhile showed the well-known changes in the kidneys with microaneurism, thickened basal lamina, mesangial proliferation, and hyaline bodies. The changes in the inner ear, especially in the region of the stria vascularis and lamina spiralis ossea to the ganglion cochleae, were rather discrete, so that no pronounced diminution of the auditory function as a result of restricted metabolism was to be expected. A loss of ganglion cells was seen in the spiral ganglion of the cochlea in correlation with ageing. There was no clear difference between diabetic rats and normal controls. Insulin deficiency diabetes causes severe changes in the vessels, as demonstrated in the kidney. In the inner ear, however, there were no comparable changes, the hearing ability of diabetic rats remaining practically normal.