1. The K(+) content of brain-cortex slices incubated in a Krebs-Ringer phosphate medium was the same whether the substrate was glucose (11mm) or pyruvate (11mm). 2. 2,4-Dinitrophenol (30mum) and ouabain (83mum) lowered the K(+) content of brain-cortex slices by approx. 40% with either glucose or pyruvate as the substrate. Triethyltin (1.3mum) lowered the K(+) content by 73% with glucose as substrate, but by only 18% when the substrate was pyruvate. 3. Quantitative estimations of six individual amino acids were made on brain-cortex slices and on the incubation medium after 75min. at 37 degrees . Some differences were found depending on whether the substrate was glucose or pyruvate. The effects of 2,4-dinitrophenol, oligomycin, triethyltin and ouabain were studied. Triethyltin caused a marked increase in the medium of each amino acid when the substrate was glucose, but not when it was pyruvate. Ouabain caused a similar increase with either glucose or pyruvate as substrate. 4. The uptake of triethyltin by brain-cortex slices was measured by using radioactive triethyl[(113)Sn]tin chloride. Various experimental conditions did not alter the uptake; these included adding glucose or pyruvate as a substrate. 5. From the author's own results and those of others it is suggested that pyruvate is an unsuitable substrate to use for the study of regulatory factors of endogenous pyruvate metabolism in brain-cortex slices. Glucose is preferable.