The normal anatomy of the pig tailed macaque (Macaca nemestrina) outflow system has until now been undocumented. Tissue from ten monkeys was investigated by light microscopy, transmission electron microscopy and scanning electron microscopy. Although the basic configuration of the outflow apparatus was similar to other subhuman primates it did differ in the following aspects: a) bundles of circumferentially orientated smooth muscle cells were present in the outer and posterior part of the trabecular meshwork near Schlemm's Canal, b) large 'atypical' endothelial cells with microfilament rich cytoplasm were present in the cribriform layer, c) numerous collagenous cords and septae were present in Schlemm's Canal which also appeared to be drained by more than the usual number of collector channels. The possible importance of these anatomical differences is discussed.