The pressure-flow relationship of the choroidal circulation of the cat was studied by the Krypton clathrate Kr 85 desaturation technique. At intraocular pressures below 20 mm Hg, the choroidal circulation was found to behave like a passive vascular bed; above 30 mm Hg, it was found to behave like an active bed with a mild degree of autoregulation. The net effect of the systemically administered vasoconstrictor agents was found to consist of an increase in the rate of flow of blood through the choroid. Systemically administered vasodilators resulted in a decrease in flow.