Restriction of both oxygen influx to the cornea and carbon dioxide efflux from the cornea by contact lenses results in adverse tissue changes. We measured the extent of hypoxia and hypercapnia at the corneal surface of 10 human volunteers during static, dynamic (blinking), and closed-eye wear of hydrogel and nonhydrogel contact lenses of different gas transmissibilities. During open-eye wear, hypoxia and hypercapnia are lower beneath lenses of higher oxygen and carbon dioxide transmissibilities, respectively (p less than 0.001). Blinking plays a significant role in alleviating corneal hypoxia (p less than 0.05), but not hypercapnia, during lens wear. In the absence of lenses, the gaseous tensions at the anterior corneal surface during eye closure were: pO2 = 37.4 +/- 20.9 mm Hg and pCO2 = 39.3 +/- 3.1 mm Hg. Closed-eye lens wear resulted in greater levels of hypoxia and hypercapnia that were directly correlated with lens transmissibility (p less than 0.001). These data form the basis of models for predicting adverse tissue changes during contact lens wear and suggest prophylactic and therapeutic clinical strategies for alleviating lens-induced hypoxia and hypercapnia.