  We study the behavior of two diferent models at finite temperature in a
$D$-dimensional spacetime. The first one is the $\lambda\varphi^{4}$ model and
the second one is the Gross-Neveu model. Using the one-loop approximation we
show that in the $\lambda\varphi^{4}$ model the thermal mass increase with the
temperature while the thermal coupling constant decrese with the temperature.
Using this facts we establish that in the $(\lambda\varphi^{4})_{D=3}$ model
there is a temperature $\beta^{-1}_{\star}$ above which the system can develop
a first order phase transition, where the origin corresponds to a metastable
vacuum. In the massless Gross-Neveu model, we demonstrate that for $D=3$ the
thermal correction to the coupling constant is zero. For $D\neq 3$ our results
are inconclusive.
  Pacs numbers: 11.10.Ef, 11.10.Gh
