In this talk we will discuss non-perturbative production of fermionic dark matter in the early universe, during the transition between inflation and radiation domination known as (p)-reheating. Specifically, I will focus on the gravitational production mechanism accompanied by the coupling of fermions to the background inflaton field. The latter leads to the variation of effective fermion mass during preheating and makes the resulting spectrum and abundance sensitive to its parameters. Assuming fast preheating that completes in less than the inflationary Hubble time and no oscillations of the inflaton field after inflation, we find an abundant production of particles with energies ranging from the inflationary Hubble rate to the inverse duration of preheating. The produced fermions can account for all observed dark matter in a broad range of parameters. As an application, we discuss the non-perturbative production of heavy Majorana neutrinos in the model of Palatini Higgs inflation.