The derivative expansion of the one-loop effective action in QED$_3$ and
QED$_4$ is considered. The first term in such an expansion is the effective
action for a constant electromagnetic field. An explicit expression for the
next term containing two derivatives of the field strength $F_{\mu\nu}$, but
exact in the magnitude of the field strength, is obtained. The general results
for fermion and scalar electrodynamics are presented. The cases of pure
electric and pure magnetic external fields are considered in details. The
Feynman rules for the perturbative expansion of the one-loop effective action
in the number of derivatives is developed.