  Dynamical chiral symmetry breaking (D$\chi$SB) is studied in the
Nambu-Jona-Lasinio model for an arbitrary combination of external constant
electric and magnetic fields. In 3+1 dimensions it is shown that the critical
coupling constant increases with increasing of the value of the second
invariant of electromagentic field $\vec{E}\cdot\vec{B}$, i.e. the second
invariant inhibits $D\chi SB$. The case of 2+1 dimensions is simpler because
there is only one Lorentz invariant of electromagnetic field and any
combination of constant fields can be reduced to cases either purely magnetic
or purely electric field.
