We have previously reported iatrogenic retinal to choroidal vein anastomosis, developed as a potential method of by-passing the site of obstruction to venous outflow in retinal venous occlusion in dogs (1), and in rats (2). In order to minimise tissue damage to the retina and choroid and increase the rate of success in these experiments, we investigated in the dog model the factors that would promote an anastomosis and compare the effects of three different power levels. A small spot size (50 microns) argon green laser beam of 514 nm at power levels of 0.5w, 1.5w and 2.5w were used. Spaced serial sections from each lesion were examined by light and by transmission electron microscopy. Morphometric measurements of the corresponding retinal pigment epithelium (RPE)/glial scar was computed using IBMPC digitising pad and sigma scan software and the extent of tissue damage at the different power levels assessed. At the lowest power level of 0.5w the damage to the retina was mild and there was an absence of anastomosis formation. At the 1.5w power level an anastomosis formed in 40% of the lesions. At the highest power level of 2.5w a 71% rate of success was obtained however, the damage to the retina tended to be severe. The results of this study also indicate that disruption of Bruch's membrane and vein rupture at the time of irradiation are essential for anastomosis formation, which may be further enhanced by necrotic tissue, retinal pigment epithelial and glial scar formation and inflammation. These findings are useful in establishing optimal conditions for the creation of a chorioretinal venous anastomosis, for consideration in human trials.