We have studied the effects of whole body cooling on phrenic nerve and diaphragmatic function in 26 patients using magnetic stimulation of the phrenic nerves with a pair of Magstim 200 HP stimulator coils during cardiopulmonary bypass. The diaphragmatic electromyogram in response to magnetic pulses was recorded with needle electrodes at two temperatures, approximately 31 degrees C (cold) and approximately 36 degrees C (warm) during the cooling or rewarming phase of hypothermic cardiopulmonary bypass. This 5 degrees C temperature change was associated with clear changes in the evoked electromyographical response of the diaphragm. Median latency between stimulus and electromyographic response was 10.1 (range 8.0-11.8) ms during cold and 8.3 (5.9-10.2) ms during warm stimulation (P < 0.001). Median duration of the muscle compound action potential was prolonged and its amplitude reduced in cold compared with warm stimulations (P < 0.01). These effects were enhanced by application of ice slush to the heart. We conclude that diaphragmatic function may be affected by mild hypothermia after cardiac surgery.