Experimental renovascular malignant arterial hypertension was produced by modified Goldblatt's procedures, in 60 rhesus monkeys, and various retinal arteriolar changes in hypertensive retinopathy were studied in detail (by serial ophthalmoscopy, and stereoscopic color fundus photography and fluorescein fundus angiography on long-term follow-up). The retinal arteriolar changes, in ophthalmoscopically visible arterioles, consisted of arteriolar sclerosis and associated changes, e.g., increased arteriolar tortuosity, arteriolar narrowing and in some animals occlusion and sheathing of the fine arterioles; we could find no evidence of localized or generalized 'spasm' in these retinal arterioles. Eyes in animals with accelerated arterial hypertension revealed focal dilatation and leakage of the retinal precapillary terminal arterioles (resulting in development of focal intraretinal periarteriolar transudates), and also occlusion of the terminal retinal arterioles (producing cotton-wool spots and associated intraretinal microvascular abnormalities). We discuss the controversial subjects of narrowing (particularly 'spasm') of ophthalmoscopically visible retinal arterioles and of fibrinoid necrosis in malignant hypertension.