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The 242m Am and 245 Cm fission cross sections in the neutron energy range 0.03 eV-20 keV were measured with a SVZ-1000 lead slowing-down neutron spectrometer at the Institute for Nuclear Research of the Russian Academy of Sciences. The procedure for analyzing the experimental data relies on the SVZ mathematical model (based on the Monte Carlo method), which makes it possible to expand the dynamical range of the spectrometer into the thermal-neutron range. The parameters of the model were chosen on the basis of the best description of the measured fission cross sections for the nuclear standards 235 U and 239 Pu.Np, Am, and Cm fission by low-and medium-energy neutrons is a very important process of transmutation nuclear wastes on the basis of reactor and accelerator technologies [1]. The difficulties of measuring the fission cross sections of these highly radioactive nuclei are due to the need to use substances that require intense neutron sources. Lead slowing-down neutron spectrometers [2-4], even though their resolution is low, are successfully used to measure the cross sections of nuclides which are difficult or impossible to investigate by other methods. In the present work, the fission cross sections of 242m Am(n, ƒ) and 245 Cm(n, ƒ) have been investigated using a SVZ-100 spectrometer [5,6].Ordinarily, the dynamic range of such spectrometers is limited below by the energy ~1 eV, below which its characteristics degrade rapidly because of the thermalization of the neutrons [3,4]. In the range E > 1 eV, the average neutron energy E(t) at the time t obeys the relationwhere K = 160-183 keV·µsec 2 and t 0 = 0.3-0.5 µsec are the slowing-down parameters.Taking account of thermalization [4] for the case of a heavy gas moderator (E >> kT) changes relation (1):where kT is the temperature of the moderator; e T = 2kT/3. Resolution degrades for long moderation time (t > 200 µsec). A theoretical estimate of the resolution ℜ = √  D E /E(t), where D E is the variance of the neutron energy distribution function at the time t, has the form [4] ℜ(E) = [(8/3A) + (kT/E)] 1/2 .