Previous studies show that choroidal blood flow is not autoregulated when intraocular pressure (IOP) is increased to raise venous pressure and lower the perfusion pressure gradient. However, the autoregulatory response to changes in mean arterial pressure (MAP) is unclear. In the current study, the perfusion pressure gradient (MAP-IOP) was altered by (1) decreasing MAP while IOP was held at 5, 15, and 25 mmHg, and (2) increasing the IOP at the prevailing MAP in anesthetized rabbits (n= 8). An occluder on the thoracic vena cava was used to vary MAP; this was monitored through an ear artery catheter. Two catheters were inserted in the vitreous to monitor and control IOP. Choroidal blood flow was measured by laser Doppler flowmetry using a slender stainless-steel probe positioned next to the retinal surface. The efficacy of autoregulation depended on the IOP. When IOP was held constant at 5 mmHg, choroidal blood flow did not fall until the perfusion pressure gradient was less than 40 mmHg. The pressure-flow relationship became progressively more linear (ie, the efficacy of autoregulation decreased) when the IOP was held constant at 15 and 25 mmHg. When IOP was varied and MAP was held constant, the pressure-flow relationship was linear at IOPs greater than 20-25 mmHg. However, choroidal blood flow was pressure independent when the IOP was less than 20-25 mmHg. Simultations using a myogenic mathematic model of the choroid gave results similar to the experimental observations. It was concluded that a myogenic mechanism may be responsible for the autoregulation of choroidal blood flow in the rabbit.