Iris circulation was investigated by fluorescein iris angiography in 33 human eyes before and after tenotomy of one or more recti for correction of strabismus. Tenotomy of one or both horizontal recti produced no appreciable circulatory disturbance in the iris, but tenotomy of superior or inferior rectus produced circulatory delay in superior temporal or inferior temporal sectors, respectively, of the iris. When tenotomies of a horizontal and one or two vertical recti were combined, the defect occurred in the region of the vertical rectus only. Blood supply of the nasal half of the iris was usually not disturbed by tenotomy of the vertical and/or medial rectus. The findings indicate that the blood supply of the iris is segmental and suggest that, in strabismus surgery, cutting the two vertical recti along with the lateral rectus may subject the eye to the risk of anterior segment ischemia. In the light of the present findings, physiological anatomy of the blood supply of the iris and role of the major arterial circle of the iris are discussed.