  A simplified Walecka-type model is investigated in a cosmological scenario.
The model includes fermionic, scalar and vector fields as sources. It is shown
that their interactions, taking place in a Robertson-Walker metric, could be
responsible for the transition of accelerated-decelerated periods in the early
universe and a current accelerated regime. It is also discussed the role of the
fermionic field as the promoter of the accelerated regimes in the early and the
late stages of the universe.
