Abstract Motor and sensory conduction velocities were measured in sural and tibial nerves of 25-week-old genetically diabetic ( db db ) mice and their nondiabetic littermates. For motor conduction velocity determination, the sciatic nerve was stimulated at the hip and the tibial nerve subsequently stimulated at the ankle while recording interosseous muscle potentials from needle electrodes placed in the foot. Sensory conduction velocities were determined by recording compound action potentials directly from sural and tibial nerves at the ankle after sciatic nerve stimulation. Control and diabetic conduction velocities were compared by Student's t test. The motor conduction velocity was reduced by approximately 20% from the control, and the distal motor latency was increased in db db mice by 22% more than the control latency. Conduction velocity was also reduced in some sensory fibers, an observation not previously reported in the db db mouse. Sensory fibers most severely affected were the faster-conducting fibers of the sural nerve, whose conduction velocity was decreased by 18% from the control. Slower-conducting sensory fibers in sural and tibial nerves were only midly affected, whereas fast-conducting sensory fibers of the tibial nerve appeared to remain normal. These data suggest that not all nerve fibers react alike to the diabetic state in the genetically diabetic ( db db ) mouse.