Estimation of the degree of stomatal opening by infiltration of liquids provides a method whereby the crop plant may be used as an indicator for timing of irrigation in field experiments where a simple routine procedure is Deeded and, as in some areas in the Sudan, fixed irrigation schedules or water balance estimates are not feasible. Infiltration techniques used by previous workers, attempts by some of them at calibration by direct microscopic examination of stomata and by stomatal resistance to air flow and investigations of the infiltration pattern in relation to changes in environmental factors, such as are found in the diurnal cycle and the irrigation cycle, are discussed.Some techniques were considered unsuitable: the infiltration of large areas of leaf becauseof difficulties in grading, the timing of infiltration because of difficulty in determining the end point and in distinguishing between entry through stomata and subsequent spread through intercellular spaces and because of possible effects of the liquid on the stomata. The technique selected employs drops of graded mixtures of liquids allowing an observation time of 2 seconds.The mechanism of infiltration is not fully understood. The difficulties in relating infiltration simply to pore diameter and to the surface tension, viscosity, and contact angle of the liquid are examined. The possibility that the contact angle of the test liquid and the slope of the wall within the stomatal pore or at the substomatal cavity may change in relation to themoisture status of the plant and the degree of stomatal opening is discussed in relation to Adam's treatment of the passage of a liquid through a pore with a constriction.