The normal eyes of 6 men and 21 rabbits were exposed to samples of lake water, one eye to a sample of pH 4.6 and the other to a sample of pH 6.3. The men's eyes were exposed for 5 minutes on four occasions a week apart, whereas the rabbits' eyes were exposed for 15 minutes either on one occasion or once a day for 7 days. In the humans neither sample of water produced symptoms or signs of an adverse effect on the external eye tissues, apart from brief conjunctival congestion after every exposure. In the rabbits the two samples did not appear, in general, to have different effects on the ocular tissues, as judged from the osmolarity and cell count of the tears, conjunctival congestion, corneal staining with fluorescein, corneal permeability and histologic features of the cornea. In a few instances differences were observed, but their pathological significance was not apparent. These data suggest that lake water of a pH as low as 4.6 may not harm healthy eyes, however, larger and broader studies are essential.