An ab externo fistulizing procedure (sclerostomy) was performed in vivo in 48 rabbit eyes using a pulsed (20 microseconds and 200 microseconds) Holmium:YAG (Ho:YAG) laser (2120 nm) and a pulsed (200 microseconds) Erbium:YAG (Er:YAG) laser (2940 nm). The laser energy was delivered via an infrared transmitting fiber in contact with the sclera, with the fiber inserted into a specially sharpened retractable cannula in order to guide it into the subconjunctival space. An additional optical fiber-fiber coupling system was required for the Er-YAG laser. A patent fistula (200-micrometers diameter) with formation of a filtering bleb and marked intraocular pressure reduction was achieved in nearly all cases. Significant intraoperative or postoperative complications occurred only with the 20-microsecond Ho:YAG laser. Trauma to the conjunctiva was as minimal as in a subconjunctival injection. The Er:YAG laser created a smaller thermal damage zone at the fistula walls and required significantly less laser energy than the Ho-YAG laser.