Several approaches to the evaluation of potential evapotranspiration (PE) are discussed from the point of view of the energy balance equation. Hourly and daily PE were evaluated for an irrigated ryegrass site near Simcoe in Southern Ontario during the summer of 1967 using the Bowen ratio and Penman solution of the energy balance equation, Thornthwaite's temperature equation and the water equivalent of the net radiation. The Bowen ratio values were used as the standard against which the other methods were compared. For the evaluations, measurements of temperature, humidity and wind profiles, net radiation, soil heat flux, screen temperature and humidity and wind run were made. Excellent results were obtained from the Penman equation (using Businger's with function) for individual hourly periods on days when previous rainfall or irrigation ensured true PE conditions. On other drier days, Penman values were consistently too large. The water equivalent of the net radiation was highly correlated with Bowen ratio values on a hourly basis but overestimated consistently. A poor correlation between Thornthwaite values and the net radiation is demonstrated on a daily basis, which indicates its tenuous connection with the energy balance equation. The relation between cumulative dry matter productivity of the ryegrass and cumulative PE is shown to be linear, but the productivity rate changed from 1 g/525 mm when the crop was irrigated to 1 g/1287 mm after irrigation had ceased.