The form and magnitude of K+ Krebs-induced contractures has been measured for the dog ophthalmociliary artery. Ring segments of the vessel were mounted in a myograph after the method of Hogestatt, Andersson and Edvinsson (1983, Acta Physiol. Scand. 117, 49-61). For low passive tensions the K(-)-induced contractures show both a phasic and tonic component, with the tonic tension component being the smaller. As passive tension is increased, the phasic component grows and plateaus whilst the tonic component continues to increase. For high passive tensions the phasic component disappears and the tonic component reduces in magnitude and the contracture is not fully reversible. Comparison of the length tension curves measured during K+ Krebs-induced contractures and Ca2(+)-free Krebs-induced relaxation shows that the active tension generated by this vessel increases with increasing passive tension until a peak active tension of 3.1 mN mm-1 is reached at an effective radius of 357 microns. For greater values of passive tension the active tension decreases. The calculated transmural pressure required to maintain the vessel at this optimal radius is 51.9 mmHg, which is very similar to the only available measurements of ophthalmic artery pressure, viz. 51.8 mmHg (Grunwald, Sinclair and Riva, 1981, Invest. Ophthalmol. Vis. Sci. 20, 564-6). The optimal passive stretch to apply to these ring segments for further pharmacological or physiological work is discussed.