To investigate the change in axial eye dimensions resulting from stretching the sclera by acute elevation of intraocular pressure (IOP). IOP was increased to 100 mm Hg for 1 hour through an intravitreal cannula, while ocular component dimensions were monitored every 10 minutes with A-scan ultrasound in anesthetized animals (10 chicks and 10 tree shrews). In addition, immunocytochemical detection of alpha-smooth muscle actin (alpha-SMA) using a monoclonal antibody was conducted in the sclera of the tree shrew and the chick. In both species, axial eye length immediately and significantly (P < 0.0001) increased on elevation of IOP to 100 mm Hg: chick to 103.9%, tree shrew to 101.2% (mean percentage of original measured at 15 mm Hg). After 1 hour of maintained pressure, chick eyes showed a further significant increase in axial length (to 108.6%), but axial length of tree shrew eyes decreased (to 100.3%) to the point that it was not significantly different from the original value at 15 mm Hg. Immunocytochemical studies of age-matched tissue demonstrated the presence of alpha-SMA-containing fibroblasts (myofibroblasts) within tree shrew but not chick sclera. Elevation of IOP caused axial elongation of chick eyes, but a consistent reduction in axial length of tree shrew eyes. The presence of myofibroblasts, demonstrated in tree shrew but not chick sclera, suggests that the reduction in axial length of tree shrew eyes may have been caused by activation of a contractile mechanism involving scleral myofibroblasts. Such a mechanism may play a role in the regulation of eye size and refractive development.