  We derive the Gell-Mann and Low renormalization group equation in the
Wilsonian approach to renormalization of massless $g\phi^4$ in four dimensions,
as a particular case of a non-linear equation satisfied at any scale by the
Wilsonian effective action.
  We give an exact expression for the $\beta$ and $\gamma_{\phi}$ functions in
terms of the Wilsonian effective action at the Wilsonian renormalization scale
$\L_R$; at the first two loops they are simply related to the gradient of the
flow of the relevant couplings and have the standard values; beyond two loops
this relation is spoilt by corrections due to irrelevant couplings. We
generalize this analysis to the case of massive $g\phi^4$, introducing a
mass-independent Wilsonian renormalization scheme; using the flow equation
technique we prove renormalizability and we show that the limit of vanishing
mass parameter exists. We derive the corresponding renormalization group
equation, in which $\beta$ and $\gamma_{\phi}$ are the same as in the massless
case; $\gamma_m$ is also mass-independent; at one loop it is the gradient of a
relevant coupling and it has the expected value.
