Potato tubers (Solanum tuberosum L. cv. Russet Burbank) were cut into slices 1 mm thick. Tissue was placed in sealed ampules in a calorimeter operated isothermally at 25 degrees C, The metabolic heat rate began to increase within 2 h after slicing, reaching a maximum after 30 h. The peak heat rate was six times that of the initial rate. Heat production was cyanide-sensitive at first while that which developed with time was largely cyanide-insensitive. The initial heat rate per oxygen consumed increased to a peak value, indicating greatly reduced energy conservation. The ratio of CO2 produced to O-2 consumed, together with the CO2 rate/heat rate indicated a change from more reduced to more oxidized substrates, i.e. from lipid to starch, Calorimetric results were in good agreement with published results obtained with other methods.