Central motor pathways were studied in 17 normoalbuminuric insulin-dependent diabetic (IDDM) patients who had been diabetic for more than 20 years, and compared with findings in 17 age-, sex-, and height-matched control subjects. The central motor conduction time was calculated from recordings of the compound muscle action potentials of the abductor pollicis brevis muscle after single transcranial and spinal root magnetic stimulation. The central motor conduction time from motor cortex to cervical spinal roots was 9.8 +/- 1.65 ms in diabetic patients and 10.1 +/- 1.48 ms in control subjects. In diabetic patients with neuropathy the central motor conduction time was 9.5 +/- 1.76 ms vs 10.1 +/- 1.56 ms in patients without neuropathy. The excitability of the motor pathways was studied by paired transcranial magnetic stimulation at interstimulation intervals of 30-1000 ms. In normal control subjects, an early facilitation of the amplitude of the compound muscle action potential at an interstimulation interval of 30 ms was found, while no facilitation was present in diabetic patients. In addition the compound muscle action potential latencies were prolonged at interstimulation intervals of 30-50 ms in diabetic patients. The changes of excitability did not correlate with the presence of peripheral neuropathy, metabolic control or diabetes duration. It is concluded that long-term normoalbuminuric IDDM patients have imparied excitability but normal central conduction time of the motor pathways.