PURPOSE: Possible toxic effects of ofloxacin on human corneal endothelia were assessed by using electron microscopy and in vitro specular perfusion. METHODS: Five pairs of corneas [with one cornea of each pair receiving balanced salt solution (Endosol; Allergan, Irvine, CA, U.S.A.) and the other receiving Endosol with 10 micrograms/ml of ofloxacin] underwent perfusion for 3 h with corneal pachymetry every 15 min followed by tissue fixation. A mean corneal swelling rate was calculated from a first-order regression line fit for each of the five experiments. The swelling rates between groups was compared by-using a paired t test. Scanning and transmission electron micrographs were examined for cellular architecture. Another group of five pairs of corneas was perfused the same way with the only difference being a test dose of 30 micrograms/ml. RESULTS: The mean swelling rate for corneas perfused with 10 micrograms/ml of ofloxacin of -3.5 microns/h was not significantly different from that of the Endosol-alone corneas at -3.0 microns/h (p = 0.71). The mean swelling rate for corneas perfused at 30 micrograms/ml of ofloxacin was -4.1 microns/h, not significantly different from Endosol-alone perfused corneas at -6.5 microns/h (p = 0.08). No consistent ultrastructural changes could be attributed to exposure to 10 or 30 micrograms/ml of ofloxacin. CONCLUSION: Human corneal endothelium can be exposed to ofloxacin at a dose of 30 micrograms/ml for a period of 3 h without adverse ultrastructural or physiologic side effects.