The dose rates from an implanted 238Pu heat source have been measured and calculated. The source material was medical grade plutonium of nominal 30-W strength. The tissue-equivalent phantoms were both a large homogeneous right-circular cylinder and a man-simulating Remab phantom. Calculated dose rates agreed to within 20% with measured values except for positions very close to the source. The reasons for discrepancies greater than this are thought mainly to arise from the uncertainties in the photon emission rate and the use of dosimeters too large for neutron measurements close to the source. In general, the agreement of the measured and calculated values is good, at least in the regions where the photons and neutrons make their greatest dose contribution for a source enclosed in a circulatory support system. It is concluded that an artificial heart device incorporating a medical grade plutonium heat source is probably acceptable to the recipient from a radiation dose standpoint independent of its acceptability as a prosthetic device.