To determine whether or not platelet noradrenaline (NA) content reflects a long-term variation of plasma NA concentration, the following experiments were carried out. During a 2-hour intravenous infusion of NA in rabbits, plasma NA concentration increased rapidly to an almost consistent level 1 h later, but platelet NA content increased linearly. One hour after the infusion, the plasma NA concentration returned to normal level, while platelet NA content decreased little and only returned to the original level about 80 h later. By the 24-hour cold exposure stimulation (4 +/- 0.5 degrees C), platelet NA content and urinary NA excretion increased significantly in rats. There was a significant correlation between the urinary NA excretion and platelet NA content. In a patient with pheochromocytoma, the high plasma NA concentration returned to nearly normal levels a few hours after removal of the tumor, platelet NA content returned to normal levels 4 days after surgery. These results indicate that the measurement of platelet NA content serves as a better index for obtaining an average variation of NA concentration and sympathetic nerve activity. The measurement of platelet NA content also provides a reliable means of diagnosing pheochromocytoma.