The majos purposes of this work were to measure the evapotranspiration in a potato field-crop under three soil moisture regimes and to correlate these values with solar radiation data and to those employing Penman's formula. The estimatives obtained by the two methods studied were significantly correlated to the evapotranspiration values measured in the various periods of the plant growth. In the first stage of the growth season, Penman's formula uppertestimated the measured values in about 25-30%, however from the stage of tuber growth to maturity, it suplied the best estimates, except in the tratment where the matric potencial of soil water reached -2.0 bars. This contributed to reduce evapotranspiration rate during all growin season. The relationship between evapotranspiration and solar radiation is presented for varius stages of the plant growth.