The ultrastructural alterations of 10 pigmented rabbit irides 15 minutes to 257 days after irradiation with a xenon arc lamp were analysed. Pulse duration was varied while irradiation power was kept constant at 3.24 W. 0.32 J (= 100 ms pulse duration) were delivered to the left eye and 3.2 J (= 1 second pulse duration) to the right side. The area of damage event was considerably smaller at the lower energy level. However depth of the crater was the same at both higher and lower levels. Cells and organelles disintegrate forming lamellar structures and coarse granules. The shape of the pigment granules remains intact. Collagen fibres disintegrate into subunits loosing their periodicity. Mesenchymal cells cover the area of damage few days after irradiation at the low energy level. A thin scar remains at the impact site. High energy irradiation shifts the equilibrium towards degenerative phenomena. Secondary degenerative processes are powerful enough to induce a total decay of the iris substance. The reactivity of the iris irradiated by a xenon arc lamp thus depends from either irradiation duration, power level or from both (energy).