Photodynamic therapy (PDT) involves photosensitizing tissue and then activating it with monochromatic light, causing necrosis. Precise control of the extent of injury should be possible by varying the energy density of the light applied to the target tissue. We tested the sensitivity of colonic tissue to PDT by injecting 10 mg/kg Photofrin II intraperitoneally in 10 rats. After 24 hr the left colon was opened and cleansed. A 1.0-cm2 area of mucosa was exposed to 630 nm (red) light produced by an argon-pumped dye laser. Pairs of rats were treated with energy densities of either 10, 20, 40, 60, or 80 J/cm2, controlled by varying exposure times. After 48 hr, we sacrificed the rats and fixed, sectioned, and stained the left colons. The depth of injury was measured with an ocular micrometer and expressed as a percentage of normal bowel wall thickness. A curve was fit to the data points by computerized nonlinear regression. The relationship between depth of injury (Y) and energy density (X) was found to fit the equation Y = 1 - aebx, where constants a = 1.15 and b = -0.0353, (R2 = 0.93, P less than 0.001). The relationship between injury and energy density is biphasic, rising rapidly from 0 to 40 J/cm2 and more slowly after this point, suggesting that colonic mucosa is more sensitive to PDT than muscularis, providing a margin of safety against perforation. Bowel perforation did not occur in this study but is predicted by extrapolation for energy densities of 100 J/cm2 or greater. These data indicate that photodynamic colonic mucosectomy is possible.