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1. A method is described for the accurate quantitative analysis of ketone bodies from small samples (0 1 ml.) of blood and urine.2. The method is based on the conversion of all ketone bodies into acetone, on the reaction of acetone with 2:4-dinitrophenylhydrazine and on subsequent colorimetric determination of acetone-2:4-dinitrophenylhydrazone in carbon tetrachloride.3. A combined reflux and distillation apparatus has been designed which ensures reproducibility in the estimation of P-hydroxybutyric acid and allows the separate and direct estimation of acetone plus acetoacetic acid and of P-hydroxybutyric acid from one sample.4. At a ketonaemic level of 40 mg./100 ml., the standard deviation of an individual reading is less than ± 0-2 mg./100 ml.

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The maintenance of differences in electrolyte composition between intracellular and extracellular fluids in kidney slices (predominantly tubular cells) is a process which requires the expenditure of metabolic energy (see review by Davies, 1954). Whittam & Davies (1954) have shown that the rates of uptake of 24Na and 42K were very much greater at 37 than at 0°C, as would be expected if the transport process was associated with a biochemical reaction rather than a physical process such as simple diffusion through a permeable membrane, on which the effect of temperature would be smaller.It has been previously suggested (Cort & Kleinzeller, 1956 Kleinzeller & Cort, 1957 a, b) that only Na is actively transported out of renal tubular cells (at the basal membrane), while K appears to behave more passively according to gradients of concentration and electrical potential across the cell membrane. An attempt has been made to examine this question further by observation of the effect of three different temperatures (15, 25 and 300 C) on the two fluxes of greatest physiological interest, the efflux of Na (Mo, Na) and the accumulation of K (Mi, K) during incubation of kidney cortex slices. (In this paper the symbol M represents amount transported, whereas M' represents the amount transported in unit time.) Change of temperature may be expected to influence the rate of transport at the cell membrane in a complex manner since permeability, active transport and potential gradient (Q) will probably all be affected. 'D cannot be independently varied in tissue slices, and has not been measured here. In order to minimize the effect of T on D a kinetic, rather than a steadystate, approach has been adopted, the rate of transport being extrapolated to zero time by means of tangents to the curves of quantity transported versus time at the three temperatures involved. Since all the slices at the start of incubation are in approximately the same metabolic state and the same ionic environment, variation of 1D with A1T should be at a minimum with a zero time approximation.