Experiments were performed to investigate the effect of cooling and warming on the relaxation to acetylcholine in isolated canine saphenous arteries. Rings of saphenous artery were stimulated with norepinephrine and cumulative concentration response curves to acetylcholine were compared at 41 degrees C, 37 degrees C, 30 degrees C, 25 degrees C and 15 degrees C. The data showed that the effect of acetylcholine is depressed by warming and a tendency to increase is observed during cooling, suggesting that muscarinic receptors may be sensitive to temperature changes.