Studies were carried out with labeled microspheres using the reference sample method in pigs. Total and regional ocular flows increased in reponse to norepinephrine and isoproterenol infusions. Increase in flows during norepinephrine infusion may be due to increased ocular perfusion pressure induced by the drug, thereby masking its reported direct vasoconstrictor effect in isolated perfused ocular vessels. Increase in flows during isoproterenol infusion occurred despite a decrease in arterial pressure, suggesting the existence of vasodilatory .beta.-adrenergic receptors in the ocular circulation of the pig.