The isotopic constitution of K present in various rat tissues, including red blood corpuscles and blood plasma, was studied 39 41 as far as the two main isotopes 39 19K and 39 19K were concerned. A special type of mass-spectrograph was used. The isotopic ratio 39 19K / 41 19K in the ashes from bone marrow and blood plasma was distinctly lower than that of ordinary mineral K as contained in K C1 (A.R.), and the 41 content of the heavy isotope 41 19K was accordingly increased. K present in ash samples from other tissues showed the same isotopic ratio as mineral K, although a minute increase occurred in exceptional cases. A kinetic mechanism was proposed, which accounted for the difference in the isotopic ratios of tissue K and plasma K.