yield potential of a stand can be defined through site quality. one way to measure site quality is using site index equations, being available different equation types (constrained and non-constrained). the objective of this study was to compare the fitness of site index equations using different base ages, and validate them along site quality and age classes in nothofagus antarctica forests of south patagonia. four height growth equations were fitted using two constrained and two non-constrained models. the equations were fitted using three base ages (30, 40 and 50 years). self-validation was done using gradients of age and site quality. lower errors were obtained when a base age of 50 years was used for all the equations. besides this, the best fitting along the age and site quality gradients were obtained with weibull, richards and newnham equations. the self-validation values of the selected equation were comparable to the fitting presented by other authors, who worked with site index models in nothofagus spp. forests. newnham equation better described the height dominant growth in n. antarctica, which presented great biological significance according to the shape of the obtained curves. hence, this model can be recommended to be used as a prediction tool for forest management planning in n. antarctica forests.