OBJECTIVE Intraocular pressure (IOP) varies with body position and previous research has indicated that most, but not all, of the variation in IOP is due to changes in episcleral venous pressure (EVP). Positional changes in other aqueous humor dynamic parameters may contribute to the change in IOP. The purpose of this study was to investigate the variation of aqueous humor outflow facility with body position changes. METHODS Healthy volunteers, aged 24 to 45 years old, were recruited for this study. Constant weight tonography was performed using a modified electronic Schiotz tonometer in two positions: seated position, 70 degrees from horizontal with neck extended until the cornea was level with floor; and supine position. A minimum of 30 minutes was allowed between the two measurements. Tonography data were fitted to second order polynomials and values for the initial steady state IOP and the outflow facility were determined using standard tables and normograms. IOP was measured using pneumatonometry. RESULTS Forty-two eyes from 21 subjects were studied. IOP in the sitting and supine positions were 17.8 +/- 1.7 mm Hg and 19.9 +/- 1.6 mm Hg, respectively, and were significantly different (P < 0.001). The mean outflow facility in the sitting and supine positions were 0.30 +/- 0.31 microL/mL/mm Hg and 0.28 +/- 0.09 microL/mL/mm Hg, respectively, and were not significantly different (P = 0.37). CONCLUSIONS Aqueous humor outflow facility measured with electronic Schiotz tonography does not vary significantly between the supine and sitting positions.