Doppler effect measurements have been made in two plutonium-fueled fast reactor assemblies at the ZPR-3 reactor. One assembly, Assembly 45A, mocked up a large plutonium-uranium monocarbide fast power breeder reactor having a 239Pu-to-238U ratio of 1:7. The second assembly, Assembly 45, had a considerably softer spectrum, produced by replacing 40% of the canned sodium of the previous assembly by graphite. Zone-loading techniques were used to achieve these mockups. Doppler measurements were made with samples containing highly enriched 239Pu, 239Pu, and 238U mixed in a ratio of 1:7, as in a large breeder reactor, and 238U in two rod diameters. Various check experiments were performed to test the validity of the measurements. The experimental results showed a strong negative Doppler response for 238U, in good agreement with theoretical estimates. The 239Pu in the mixed isotope fuel composition gave only a small positive Doppler contribution. A separate measurement for 235U in the power breeder spectrum was positive and in reasonable agreement with theory. The 239Pu results were the most interesting. Three degrees of increasing “hardness” of neutron energy spectrum incident upon the 239Pu Doppler element were generated by Assembly 45, Assembly 45A, and Assembly 45A with natural B4C surrounding the Doppler element, respectively. The experimental positive 239Pu Doppler effect reactivity change was much smaller than calculated for the softest spectrum. The agreement between theory and experiment improved significantly, however, as the neutron energy spectrum hardened.