The effects of low power (1-2 Watts), long exposure (30-400 seconds), interstitial Nd YAG laser therapy on dimethylhydrazine induced rat colonic neoplasms and normal rat colon have been studied. After a single exposure with appropriate laser parameters, dimethylhydrazine induced rat colonic neoplasms underwent coagulative necrosis, sloughed off over a four day period, and left an ulcer which healed within 28 days. Inadequate laser energy resulted in incomplete tumour necrosis whilst excessive laser power or energy increased the likelihood of perforation. Treatment of normal colon with 1 Watt for 30 seconds or longer resulted in coagulative damage which healed by granulation. Mean colonic bursting pressures were significantly decreased one hour after treatment with 1 Watt for 75 or 100 seconds compared with untreated colon (p less than 0.05 and p less than 0.001 respectively) but not in colon treated with 1 Watt for 30 or 50 seconds. In animals treated with 1 Watt for 100 seconds mean bursting pressures were significantly lower than untreated animals when the animals were killed two, four, and seven days after lasering (p less than 0.001 in each case) but not in animals killed at 11, 17, or 21 days. The technique may be of value in the treatment of some inoperable colorectal cancers and sessile polyps in man.