1. Mitochondria isolated from the ventricles of Cavia porcellus, Mesocricetus auratus, Rattus rattus and Mus musculus accumulate calcium in an energy-linked process. 2. Measurements of the initial velocity of accumulation by dual-wavelength spectrophotometry show species-dependent variation, with Cavia demonstrating the fastest rate and Rattus the slowest rate of accumulation. 3. Total amount of calcium accumulated per unit protein varied in a manner similar to rate of accumulation, with Cavia accumulating the greatest amount of calcium and Rattus the least.