The effects of intravitreal and intracameraJ administration of endothelin-1 were studied in the anaesthetized cat. Intravitreal injection of 0.4 nmol endothelin-1 induced a 34±10% (n=5; P<0.05) reduction in retinal blood flow in the experimental eye compared to the control eye 80 minutes after the injection. Blood flow in the ciliary body, iris and choroid remained unaffected by the injection of endothelin-1.Intracameral administration of endothelin-1 at pmol doses caused a reduction in pupil size, an increase in the aqueous humor protein concentration (indicating a break-down of the blood-aqueous barrier) and an increase in the concentration of prostaglandin E2 in the aqueous humor. The effect of endothelin-1 on pupil size was abolished by indomethacin pretreatment, indicating that this effect was mediated by arachidonic acid metabolites.