In the context of the core instability model and of the oligarchic growth of solid protoplanets, we present a set of full evolutionary calculations of in situ giant planet formation. The growth of the core and the planetesimal accretion rate were coupled in a self–consistent way to the envelope's evolution. Several simulations for the formation of a Jupiter–like object were performed taking into account various surface densities for the protoplanetary disc and different sizes for the accreted planetesimals.