Brain uptake of antipyrine, water and ethanol was studied in rats under normo-, hypo- and hypercapnic conditions. Brain uptake of D- and L-lactate was studied in normal rats. The uptake was determined with the Oldendorf method, using single common carotid arterial injections of a mixture of -14C-labeled test substance and tritiated reference substance. The results demonstrate that L-lactate is taken up by the brain in significant amounts. The results also demonstrate marked differences in the uptakes of antipyrine, water and ethanol. The brain uptake of antipyrine is lower, the brain uptake of ethanol higher, than of water. The brain uptake of all 3 substances was shown not to be affected by changes in cerebral blood flow, although a decrease of brain uptake of antipyrine relative to ethanol was observed during hypo- and hypercapnia.