Using a closed chamber microelectrode system, the tear pH levels of sixteen subjects were monitored throughout the waking extent of five days. In addition to the absolute mean pH differences found among subjects, diurnal patterns of pH change could be identified for the majority. The amplitudes and periods of these cycloid patterns, however, were distinctive to each individual. Also, tear pH levels following periods of prolonged eye closure were found to be notably more acid than those associated with the waking hours.