Siesjo, B. K. and W. 0' B. Thompson. The uptake of inspired 14C0, into the acid-labile, the acidsoluble, the lipid, the protein and the nucleic acid fractions of rat brain tissue. Acta physiol. scand. 1965. 64, 182-192. -The rate of incorporation of14C0, into the acid-labile, the acid-soluble, the protein, the lipid and the nucleic acid fractions of brain tissue was studied in rats, exposed to a constant concentration of I4CO, in the inspired air for periods varying between 2 and 120 min. At the end of each exposure the head of the animal was frozen in situ with liquid nitrogen, and the I4CO, activity in the acid-labile, diffusible CO, fraction of the tissue was determined by means of a liquid scintillation technique after previous absorption of the CO, in Hyamine-1OX. For the determination of the I4CO, activity in the organic tissue fractions, these were digested in in Hyamine-1OX before counting. The total activity of "CO, in the tissue increased roughly exponentially during the first 60 min, after that time the increase was roughly linear. The corporation of "CO, into the acid-labile tissue fraction (HCO;, CO,) could be approximately described as an inflow into a two-compartment system, equilibrium being reached after about 45 min. The uptake of 14C0, into the acid-labile fraction of cortical tissue was somewhat more rapid than into that of the whole brain. The incorporation of '4CO, into the organic fractions of the tissue, i. e. the difference between the total uptake and the uptake into the acid-labile fraction, proceeded at a rate which was linear with time. At 15, 30, 60 and 120 min 8, 13, 21 and 32 per cent, respectively, of the total I4CO, present in the tissue was recovered in the organic fractions. Most of the organic '*CO, was recovered in the acid-soluble fraction, which contained about 85 per cent of the organically bound W O , , while the remaining activity was found predominantly in the protein fraction. The lipid and the nucleic acid fractions were about equally labelled, in none of these fractions did the 14C0, activity ever exceed 1 per cent of the total "CO, activity in the tissue. The results obtained lend support to the assumption that bicarbonate ions, or hydrogen ions, are actively transported between brain tissue and blood plasma.Present address: