The autoregulatory capacity of the human retina is well documented, but the pressure-flow relationship of the human choroid is still a matter of controversy. Recent data, using laser Doppler flowmetry to measure choroidal blood flow, indicate that the choroid has some autoregulatory potential, whereas most data using other techniques for the assessment of choroidal hemodynamics indicate that the choroidal pressure-flow curve is linear. We used a new laser interferometric technique to characterize choroidal blood flow during isometric exercise. Twenty healthy subjects performed squatting for 6 min during normocapnia and during inhalation of 5% CO2 and 95% air. Ocular fundus pulsation amplitude, flow velocities in the ophthalmic artery, intraocular pressure, and systemic hemodynamics were measured in 2-min intervals. To gain information on choroidal blood flow fundus pulsation amplitude was corrected for changes in flow pulsatility using data from the ophthalmic artery and for changes in pulse rate. Ocular perfusion pressure was calculated from mean arterial pressure and intraocular pressure. The ocular pressure-flow relationship was calculated by sorting data according to ascending ocular perfusion pressure values. In a pilot study in 6 healthy subjects comparable ocular pressure flow relationships were obtained when choroidal blood flow was assessed with the method described above and with laser Doppler flowmetry. In the main study isometric exercise caused a significant increase in mean arterial pressure (56%, P 69% during normocapnia and 70% during hypercapnia. Our data indicate that during isometric exercise the choroid has a high capacity to keep blood flow constant despite changes in perfusion pressure and that this pressure-flow relationship is not altered by moderate changes in arterial carbon dioxide levels.