The rf characteristics of double-barrier resonant-tunneling structures are analyzed in the small-signal approximation through a rigorous solution of the time-dependent Schrodinger equation. One of the characteristics studied is the negative dynamic conductance in the terahertz frequency range. The analysis is carried out for both monoenergetic and shifted Fermi distributions of the electrons incident on the structure. Results are reported on the modulus and phase of the transmitted and induced currents as a function of the potential well of the double-barrier resonant-tunneling structures, the electron energy, and the frequency of modulating field. It is shown, in particular, that the rf boundary of the region, in which a negative dynamic conductance exists, may be significantly higher than the value found from the half-width of the resonant level.