Rats were anaesthetized with halothane and artificially ventilated. After bilateral nephrectomy and implantation of arterial and venous catheters, arterial plasma pH (pHe) was adjusted by infusion of HCl or sodium bicarbonate to 7.2, 7.4, or 7.5 and kept constant throughout the experiment. The distribution of tris between intra- and extracellular compartments was determined as a function of time up to 24 h after infusion of 14C-labelled tris and 3H inulin in five skeletal muscle groups, heart, liver, spleen, and brain tissue. The following results were obtained: Tris diffuses very slowly into the intracellular space of the investigated tissues. For different arterial plasma pH, the intracellular tris concentration is quite different. It rises more rapidly and reaches higher levels in alkalemia. Five different skeletal muscle groups showed the same rates of rise of intracellular tris at the same pHe. Tris diffuses almost immediately into liver cells, the rates being slower in spleen, heart, skeletal muscle and brain, in that order. Only in the liver did intracellular tris concentration reach a steady state, at levels higher than theoretically predicted, suggesting that ionic tris also is permeable. In other tissues, lack of steady states at the end precluded similar conclusions. It can be concluded that the clinical importance of tris therapy is in its elimination of H+ ions from the extracellular space and in the generation of bicarbonate that then penetrates the intracellular compartment.