As there is no epithelial barrier between the anterior chamber and the ciliary muscle, aqueous humour may freely pass between the ciliary muscle bundles into the supraciliary and suprachoroidal spaces, from which it is drained through the sclera. This uveoscleral outflow of aqueous humour accounts for 40-60% of the total outflow in monkeys, whereas it is considerably less in (3-8%) in cats and rabbits. Direct measurements in human eyes have suggested that less than 15% is drained by the uveoscleral routes. However, indirect calculations have given a value of about 35% in young adults and 3% in elderly persons (> 60 years). Under normal conditions, in monkeys, the uveoscleral outflow is insensitive to changes in the intraocular pressure, but cyclodialysis and experimental uveitis increase the uveoscleral outflow and make it more pressure sensitive. The uveoscleral outflow is decreased by contraction (pilocarpine) and increased by relaxation (atropine) of the ciliary muscle. Thus, changing the tone of the ciliary muscle may redistribute aqueous humour between the conventional and uveoscleral outflow routes. Prostaglandins decrease the intraocular pressure by increasing the uveoscleral outflow. Two mechanisms seem to contribute to this effect: relaxation of the ciliary muscle and changes in extracellular matrix, causing decreased resistance in the uveoscleral outflow routes.