Genetic by environment interactions for post weaning traits were studied in a local breed of cattle, well adapted to tropical conditions. After weaning, 444 beef calves of both sexes were separated within two management systems, either in intensive fattening or at pasture. The traits analysed included weights at standard age, of 365 days (W12), 455 days (W15) and 545 days (W18), and post weaning growth rates from weaning until 15 months (PWG15) or 18 months (PWG18). (Co)variance components for all characters were analysed using restricted maximum likelihood (REML) methodology. Different bi-variate animal models were studied, involving separate traits measured one in intensive fattening and the other at pasture. Very low correlations between growth traits obtained in both management systems were observed. These results indicates that there exists an important interaction between genetic merit and environment for post weaning growth traits, and indicate that it may necessary to take into account the real management conditions for the selection on such traits in tropical conditions.