The purpose of this investigation was to study the use of the Relative Water Content of leaf tissues and the Refratometric Index of cell sap, for evaluate the water need of the tomato plant. Two field trials were conducted, during the summer and during the winter. Three treatments were used replicated seven times, in which water was supplied by the furrow irrigation method. The soil moisture was determined by the gravimetric method and the respectives water potentials by the pressure membrane apparatus. The treatments reported to three levels of soil water potential, 0,7; 3,0 and 15,0 atm. The data obtained were used for correlate the Relative Water Content and the Refractometric Index with the respectives soil water potentials. The results obtained led to the following main conclusions: a) Soil water potentials above 3,0 atmosphers resulted in better yeld. b) The readings of the Relative Water Content Correlated with respectives soil water potentials, provided a useful datum on the control of the tomato plant. c) The readings of the Refractometric Index Correlated with the respectives soil water potentials showed a glight relationship on the control of irrigation of the tomato plant.