To determine the role of endothelin (ET)-ETA receptor mediation and endogenous production of endothelin-1 (ET-1) in the retinal response to hyperoxia. Brown-Norway rats (n= 30) were injected intravitreally with an ETA receptor-selective antagonist, BQ-123, and an inhibitor of ET-converting enzyme (ECE), phosphoramidon, and were exposed to a 100% oxygen breathing mixture. Control rats underwent intravitreal injection of vehicle alone (2.5% Emulphor in phosphate-buffered saline). The retinal hemodynamic responses were analyzed using video-based fluorescein angiography (VFA) methodology. Baseline measurements were made with the animals breathing room air, and this was followed by intravitreal injections of the above agents. Subsequent VFA measurements were made after 5, 10, and 15 minutes of breathing 100% oxygen. The 10 rats injected with vehicle alone demonstrated the expected retinal response to hyperoxia, with significant (P< 0.001) vessel constriction (18%+/-5%), an increase in retinal mean circulation time (0.84+/-0.13 seconds in room air and 1.59+/-0.27 seconds in 100% oxygen), and a decrease in blood flow (110.7+/-19.2 pixel2/second in room air and 41.9+/-9.0 pixel2/second in 100% oxygen), compared to values measured during room air breathing. The hyperoxic response in rats (n= 9) injected with 10 (-4) M BQ-123 was significantly (P< 0.001) blunted compared to the group injected with vehicle alone. In contrast, intravitreal injection of saralasin, a specific angiotensin II receptor antagonist, had no significant effect on the retinal response to hyperoxia (n= 5). Intravitreal phosphoramidon (10 (-3) M, n= 6) injection also resulted in a significantly (P< 0.001) blunted circulatory response to hyperoxia compared to rats injected with vehicle alone. This blunted response after ECE inhibition was comparable to that measured after ETA receptor antagonism with BQ-123 injection. These results demonstrate that the enhancement of ET-1 action, possibly caused by the activation of ECE, plays a primary role in regulating the retinal hemodynamic response to hyperoxia.