By the perturbation calculation within the Hartree‐Fock approximation, the free energy of electrons in metals is expanded as a power series of deviations of electron and spin densities from these in the ferromagnetic state up to the fourth powers. From the expansion, the wavevector‐dependent dielectric constant and spin susceptibilities are calculated. It is shown that in ferromagnetic itinerant electrons the external electric (magnetic) field induces not only an electron (spin) density but also a spin (electron) density. The discontinuity of the longitudinal spin susceptibility at zero wavevector in the case of a &dgr;‐function type interaction has been explained by the nature of interaction. Coefficients in the expansion are calculated at 0°K in the cases of sinusoidal distributions of electron and spin densities and of a helical distribution of spin density in a paramagnetic gas, where the effective mass approximation and the Hubbard approximation for the exchange terms are used. It is found that the coefficients of the fourth powers of these density waves diverge at a wavenumber 2kf, wherekfis the Fermi momentum divided by ℏ.