The in vivo hemodynamics of the ciliary process vasculature In the albino rabbit were studied with an Improved technique of Intraocular microendos-copy. The spreading pattern and velocity of the dye front has been observed in defined vessel segments following intracarotid Injection of Evans Blue.In the anterior portion of the ciliary processes a“thoroughfare channel”(bypass) was found leading from the arteriolar tree directly Into the marginal venule bypassing the capillary network of the ciliary processes. This bypass is characterized by a higher blood flow velocity (1.5–3mm/sec) than was found for the capillary network (0.8–1 mm/sec). A stoppage of blood flow was observed in the capillary network at 45–50mmHg after step-wise elevation of intraocular pressure (IOP): blood flow was not stopped in the thoroughfare channel until IOP values of 55–60mmHg were reached. Intraarterial administration of vasoconstrictive agents could lead to a complete stoppage of blood flow in the capillary net whereas the marginal route often remained patent.