In 25 cat eyes 0.7 mL of air and in 11 cat eyes 0.7 mL of balanced salt solution were injected into the anterior chamber after paracentesis. This air persisted for three to four days, and specular microscopy was performed before and two months after air injection. There was an overall significant decrease in the endothelial cell density as a percentage of the preinjection density for the eyes with air when compared with those with balanced salt solution (102% +/- 7% for balanced salt solution and 93% +/- 11% for air). Thus, while air is generally well tolerated by the corneal endothelium, it is a toxic material in the anterior chamber and probably can cause endothelial damage. If not otherwise indicated and if safe to do so, it seems best to avoid its use or to replace it with physiologic solutions after its use.