SUMMARY We studied ocular surface pH in 161 subjects. The mean pH for 133 normal volunteers was 7* 11, SD 1*5. We found that older women had a more alkaline pH than other subjects, that the pH shifted from acid to alkaline during the day, that one hour of eyelid closure caused an acid shift in pH, and that pH recovered to baseline values within 40 minutes after acid drop instillation. We explored the mechanism of pH regulation, and we believe that pH changes could affect contact lens toleration, drug effectiveness, and clinical signs in disease processes.Using the direct contact microelectrode we have studied ocular surface pH in the inferior lateral fornix. Shifts in pH could have therapeutic and diagnostic significance. Both antibiotic effectiveness'' and cell viability' are affected by pH.pH changes may also help in understanding mechanisms of disease processes, since rosacea,4 allergy, and bacterial infection5 alter pH. Even toleration of soft contact lenses might be related to pH, because lens hydration decreases in acid pH."We present pH findings on 322 eyes (161 subjects) and explore mechanisms for the maintenance of, as well as deviations from, the steady state pH. Materials and methodsSubjects' age, sex, race, topical and systemic medication, and time of day were recorded. Patients taking substances known to be excreted in tears (methotrextate, aspirin, ampicillin, bacampicillin7 ") were excluded from the study. Informed consent was signed by all subjects.Both eyes were tested. The eye to be measured first was selected at random by coin toss. A digital readout (Corning 125) pH meter was standardised with pH 7 and pH 10 solutions and adjusted to read samples at 32°C"' before each measurement. A microelectrode pH probe (Microelectrodes, Inc. 410) was placed in the lateral inferior fornix (Fig.

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