OBJECTIVE To determine the change in intraocular pressure (IOP) due to postural changes in young healthy volunteers. METHODS Intraocular pressure was measured using a calibrated Pulsair noncontact tonometer in both eyes of 25 female volunteers in a sitting position and after 1, 3, and 10 minutes in a supine position. In the second part of the experiment (a 7-day -6 degrees head-down tilt [HDT]), IOP (at 8 am, 12 am and 6 pm) and corneal thickness (12 am) were monitored in 8 female volunteers before, during, and after the HDT period. Blood pressure, hematocrit, plasma volume and osmolality, and plasma catecholamines concentrations were also measured. RESULTS Intraocular pressure was significantly higher in the supine position (16.1 +/- 3.6 mm Hg) than in the sitting position, with a mean pressure difference of 2.23 +/- 2.9 mm Hg after 1 minute, 0.9 +/- 3 mm Hg after 3 minutes, and 1.9 +/- 3.8 mm Hg after 10 minutes in a supine position (P < 0.001). During the period of HDT, IOP values decreased significantly on the fifth day (13.3 +/- 1.6 mm Hg, P = 0.03) and the seventh day (12.7 +/- 1.7 mm Hg, P = 0.02) when compared with IOP in the supine position (14.26 +/- 2 mm Hg). The corneal thickness increased significantly (P < 0.0001) at day 5 (549.25 +/- 48.7 microm) and day 7 (540.31 +/- 46.9 microm) compared with baseline (532.45 +/- 38.6 microm). Two days after the end of the HDT bedrest, the mean supine IOP significantly increased (14.1 +/- 1.8 mm Hg, P = 0.003) and corneal thickness was similar to that found at baseline. The mean decrease of IOP was positively correlated with that of the plasma volume (-10%, r = 0.61, P = 0.02) and negatively correlated with the mean rise of hematocrit (r = -0.5, P = 0.07), variables that are considered to be indirect measures of plasma dehydration. CONCLUSIONS During a 7-day HDT bedrest experiment in healthy women, eyes seemed to compensate the moderate rise of IOP described between a sitting and a supine position, and exhibited a slight and progressive average decrease of 1.3 mm Hg. These physiological modifications could be related to an ocular dehydration or to systemic cardiovascular and hormonal variations during bedrest.