Subconjunctival fibrosis is one of the major causes of most bleb failures. To inhibit the wound-healing process and to achieve a better intraocular pressure (IOP) lowering effect as well as bleb formation, we performed a sclerostomy using a THC:YAG laser (thulium, holmium, and chromium-doped yttrium-aluminum-garnet crystal) and a subconjunctival injection of mitomy-cin-C given 24 h or 5 days before a filtering operation or soon after the filtering process in rabbits. A 1-mm conjunctiva wound was made 12 mm away from the sclerostomy site to allow for entry of an optic probe for delivering energy. Forty-eight rabbits were divided into four groups. Group I received a subconjunctival injection of 0.2 ml of 0.2 mg/ml of mitomycin-C 24 h before the operation. The same dosage of mitomycin-C was given to Group II 5 days before the operation and to Group III immediately following the operation. Group IV served as a control, and only 0.2 ml of normal saline solution was given 24 h preoperatively. One eye of each rabbit was randomly selected for the experiment, while the other eye served as a control. The bleb lasted 30.9 $pM 15.7 days in Group I, 16.0 $pM 6.4 days in Group II, 15.5 $pM 6.5 days in Group III, and 2.3 $pM 0.7 days in Group IV. The IOP lowering effect was parallel to bleb formation. The results demonstrate that a subconjunctival mitomycin-C injection given 24 h before a filtering operation is more effective in prolonging the filtering effect produced by the THC:YAG laser than one given postoperatively.