  It is known that for nonlinear electrodynamics the First Law of Black Hole
Mechanics holds, however the Smarr's formula for the total mass does not. In
this contribution we discuss the point and determine the corresponding
expressions for the Bardeen black hole solution that represents a nonlinear
magnetic monopole. The same is done for the regular black hole solution derived
by Ayon-Beato and Garcia, showing that in the case that variations of the
electric charge are involved, the Smarr's formula does not longer is valid.
