Central neutron spectra were measured with proton-recoil proportional counters in a number of uranium- and plutonium-fueled fast reactor assemblies. The energy range of the measurements is from 1 keV to 2 MeV. Short descriptions are included of the experimental technique and of the correction schemes necessary to remove inherent systematic errors in this method of spectroscopy. The measured neutron spectra are compared with homogeneous fundamental- mode calculated spectra. ENDF/B Version I data were used. In general, the agreement between the measured and calculated spectra is good. The comparisons have revealed some systematic discrepancies which are believed to be outside of experimental error near neutron scattering resonances and at low and high energies. These differences are beyond those associated with heterogeneity effects introduced by the plate loadings of the cores.
