The reaction of the precorneal tear film of the human eye was determined non-invasively by instilling pyranine, a pH-sensitive fluorescent dye. The mean value was 7.83 (S.D. +/- 0.10) and it takes up this value immediately on opening the eye after the lids had been kept closed. The HCO3 system seems to be responsible for only a portion of the buffering power of the tear film. When a drop buffered to pH 6.4 with 0.075 M PO4 was instilled, the tears returned to their normal value in about 7 min, consistent with the washout times of solutes in the conjunctival sac. A pH of 7.3 was established in the tear fluid behind contact lenses either gas permeable or impermeable, probably as a result of their restricting the loss of CO2 from the eye.