Calculations of fission product decay heat from 235U and from 239Pu based on the nominal values prepared for use in the ANS 5.1 Draft Standard (1978) are in very good agreement with measurements for cooling times from 5.0 × 104 to 1.8 × 107 s. This good agreement for 239Pu tends to support the 239Pu experimental values of Dickens et al. rather than those of Yarnell and Bendt for cooling times <105 s.