To study the effects of exposed ocular surface area and blink rate on tear dynamics by the measurement of tear evaporation under a range of conditions. Tear evaporation was measured in three gaze positions in 15 normal volunteers, and the ocular surface area was determined as a function of the width of the palpebral fissure. In 17 normal volunteers, the effect of blink rate on tear evaporation was assessed. The ocular surface area was 1.2 +/- 0.27, 2.2 +/- 0.39, and 3.0 +/- 0.33 cm2 with patients looking down, ahead, and up, respectively. The corresponding tear evaporation rates per eye were 7.0 +/- 3.5, 17.6 +/- 6.6, and 23.7 +/- 6.3 x 10(-7) g/s, respectively. The tear evaporation per square meter also increased proportionally with the ocular surface area. When the blink rate was changed from 10 to 60 per minute, the tear evaporation did not change in those individuals with evaporation rates more than 7.8 x 10(-7) g/s per square centimeter, whereas it did increase with the blink rate in those whose evaporation rates were lower. Ocular surface area and blink rate affect tear dynamics. Moderate palpebral fissure width and blink rate are necessary for the prevention of desiccation of the ocular surface.