The effect of filtering operations on 133 eyes was investigated by fluorescence photography after intracameral injection of fluorescein. The following drainage mechanisms could be found: 1. the transconjunctival route, 2. the diffuse resorption through degenerated veins, 3. bulkflow through lymphatic vessels, 4. bulkflow through atypic aqueous veins, 5. outflow through normal aqueous veins. A filtering bleb develops, if the transconjunctival route and the diffuse resorption is predominent. Newly incorporated veins and lymphatic vessels enable the drainage of the aqueous humour without a visible bleb. Most important for the formation of these vessels is the surgical technique. Therefore it is not only necessary to cover the fistula with a scleral flap but also to gain a physiological intraocular pressure immediately after the operation. If the tension is too low in most cases a filtering bleb will form. If it is at a physiological level (i.c. above the episcleral vein pressure) new vessels can develop and drain the aqueous humour by bulkflow from the scleral fistula and that means a good pressure regulating effect without a filtering bleb.