The effects of dopamine and dobutamine on plasma potassium were investigated in dogs using continuous, multi-channel, intravascular ion-selective potassium electrodes situated in the aorta and abdominal and thoracic inferior vena cavae. Doses of 10 and 30 micrograms kg-1 min-1 of each drug were used, and the effects on potassium compared with isoprenaline 0.07 and 0.2 microgram kg-1 min-1. Both the drugs caused a biphasic pattern of potassium change consisting of an initial small rise in the potassium level, followed by a sustained period of hypokalaemia. The changes were greater with the higher dose of each drug compared with the lower dose, but there were no significant differences between the drugs. Comparison of the potassium changes between the three vascular sites studied suggested that the rise in potassium may be a result of release of the ion from the liver, and that the liver may also be the principle site of potassium uptake during the hypokalaemic phase.