The active and passive wall tension--internal circumference (diameter) relations of isolated ring segments of bovine retinal small arteries (i.d. ca. 200 microns) were studied in vitro using an isometric myograph. The active tension of the vessels (when fully activated with 10(-5) M 5-HT and 10(-5) M PGF2 alpha in high K(+)-saline) reached a maximum (1.2 N m-1) at an internal circumference, L0, where the passive tension (measured in Ca2(+)-free solution) were 0.89 N m-1. Below and above L0 the active tension fell linearly, the zero tension intercepts being 0.48 L0 and 1.61 L0. The passive wall tension rose exponentially as a function of the internal circumference (diameter) and was 43 times higher at 1.4 L0 than at L0. A small transient spontaneous contraction was observed after rapid stretch of the arteries. Papaverine and Ca2(+)-free solution did not change the wall tension of vessels stretched to L0, indicating absence of stable spontaneous myogenic tone in the vessels. Histological examination showed that the retinal artery media on average had three smooth muscle cell layers which in total were 12 microns thick. The media/lumen ratio was 7%. Smooth muscle cell volume was 1228 micron3, constituting about 72% of the media volume. The active wall tension--internal circumference relation and morphology of bovine retinal small arteries is generally equal to that of arteries of similar size from other species, but active force generation of the retinal vascular smooth muscle cells is considerably lower.