Thornthwaite and Blaney-Criddle's methods were used to estimate the evapotranspiration rate in a potato field-crop under three soil moisture regimes. The evapotranspiration data obtained by the soil-water balance method were well correlated with those obtained by the two empirical methods, manly from the stage of tuber growth to maturity. Blaney-Criddle's consumptive use factors supplied the higher correlation coefficient and presented monthly consumptive use coefficients very approximate to those observed in another sites, whereas Thornthwaite's formula markedly underestimated the measured values during all lenght of the growing season. This fact could not be justified by the thermal lag. The lack of precision of the estimates observed in the present field-test seems to involve the parameters and constants of the Thornthwaite's formula. At plant emergence and in the first phase of stolon formation, the correlation coefficients were lower than for the other stages, mainly when Thornthwaite's formula was used.