The organization of F-actin microfilaments in the vascular endothelium of the porcine retina was studied in situ using rhodamine phalloidin labelling and fluorescence microscopy. A comparison was made between arterial and venous endothelial-cell microfilament distribution. The arterial cells in straight segments, bifurcations and branch points were elongated with their long axis in the direction of flow. Venous endothelial cells, on the other hand, were ellipsoid to rhomboid in shape throughout. F-actin was localized at the periphery of both arterial and venous endothelial cells. Prominent central microfilament bundles, similar to in vitro stress fibres, were oriented parallel to the long axis of arterial cells but were rarely present in venous cells. Only occasional venous endothelial cells contained short central actin filaments which were mainly in the venules. Central microfilaments were not identified in pre-capillary, capillary, or post-capillary endothelial cells. Incubation of the retinal organ cultures for 24 hr resulted in loss of the central microfilaments while peripheral staining persisted. Short-term incubation of the retinas in organ culture with low-dose cytochalasin B resulted in disruption of the central microfilaments while the peripheral actin microfilaments remained intact. The central microfilament bundles may reflect an adaptive response to arterial blood flow and may indeed be a sensitive dynamic system reflecting the influence of environmental factors on endothelial cells.