Neutron emission rates and neutron spectra were determined for two238Pu power sources: SNAP 27-1, a 1482-W plutonium dioxide source for the Apollo Space Program, and HP 15-2, a 15.84-W plutonium metal source for the Artificial Heart Program. The measurements were made with a single stilbene crystal fast-neutron spectrometer and a long counter. The specific neutron yields were (2.2±0.1)×104n/sec per gram of238Pu for SNAP 27-1 and (3.9±0.2)×103n/sec per gram of238Pu for HP 15-2. Neutrons from these sources are due to spontaneous fission of238Pu, neutron-induced fission of plutonium, and (α, n) reactions. The contributions to the two spectra due to neutrons from each of these reactions are shown. Factors for converting from neutron fluence to dose equivalent (whole body) were calculated to be (3.34±0.11)×10−5mrem per n/cm2for SNAP 27-1 and (3.13±0.15)×10−5mrem per n/cm2for HP 15-2.