We observed the course of diabetes in 23 genetically diabetic mice (C 57 BL/KSJ mdb) and in 25 controls (C 57 BL/KSJ). Diabetes was not treated. Histological changes of pancreas, aorta, kidneys, and inner ear vessels were noted. Auditory function was tested by means of the pinnal reflex of Preyer for a frequency range between 4000 and 20000 cps. The diabetic mice developed partly a mild diabetes associated with weight gain, and partly a severe diabetes with a loss of weight. Histologically, the diabetic mice showed (dependent on the duration and degree of severity of the diabetes) progressive mesangial sclerosis and thickening of basal laminas of the glomerula, as well as progressive slight to moderate thickening of the vascular walls of all inner ear vessels. The hearing ability of diabetic mice and of controls diminished with age to a comparable degree. The diabetic mice showed a slightly higher hearing loss for high frequencies, which did not correlate with the extent of thickening of inner ear vessel walls.