A quantitative investigation, using transmission electron microscopy, was conducted to determine the relative incidence of nuclei, giant vacuoles and their pores, non-vacuolar transcellular channels and minipores in the endothelium lining the trabecular aspect of Schlemm's canal at various levels of intraocular pressure in the range 0--50 mmHg. The eyes of twelve rhesus monkeys were used and prior to intracameral fixation, the intraocular pressure was maintained at the appropriate level for one hour. The results obtained from a single section analysis showed that the incidence of giant vacuoles, vacuolar and non-vacuolar transcellular channels and minipores could be related to pressure. An analysis of serial reconstructions of samples of the vacuoles indicated that vacuolar transcellular channels became more numerous and more enlarged as pressure was increased. We concluded that the transcellular pore system serves as a major outflow route for aqueous humour.