Five continuous records of arterial plasma K were obtained from 3 normal subjects during brief periods (5-7 min) of exercise (100 W). In 2 of these subjects, hepatic venous blood samples were withdrawn at 0.5-1.0 min intervals, and were analyzed in vitro for plasma K. Arterial plasma K rose rapidly at the start of exercise, from 3.8 .+-. 0.3 mmol/l (mean .+-. SD) to plateau levels of 5.4 .+-. 0.1 mmol/l. One of the above subjects and a further subject were studied after .beta.-blockade with propranolol. This resulted in an exaggerated rise in arterial plasma K during exercise. Hepatic venous K measurements indicated that the liver probably had little effect on K changes during exercise. The changes in arterial plasma K during exercise are rapid and substantial. If transmitted to the extracellular fluid, these changes would alter cell transmembrane potential, and might, as a result, alter receptor sensitivity.