We have developed a gas liquid chromatographic assay for plasma acetate which detects 0.01 mM and has a standard deviation of less than 16% of the mean at 0.05 mM. The acetate concentration of venous blood obtained from normal subjects was 0.025 +/- 0.002 mM. The value was not significantly different from the venous acetate concentration of dialysis patients, prior to a dialysis treatment. The acetate concentration of arterial plasma obtained from dialysis patients prior to a hemodialysis treatment was 0.048 +/- 0.006 mM. This was significantly greater than the venous concentration of 0.027 +/- 0.002 (p less than 0.01). This arteriovenous difference suggests that acetate is utilized peripherally. At the onset of hemodialysis, acetate concentrations rapidly rose and, in all but one case, reached a steady state value in the range of 1.5-5.1 mM. At the termination of dialysis, acetate concentrations fell rapidly, usually achieving baseline values within one hr. The rate of decrease was not a simple logarithmic function, suggesting that acetate metabolism is not solely a first order function. Furthermore, acetate disappearance was more rapid in the dialysis patients than in the normal subjects. We wish to emphasize that the present data, which were obtained on a small number of subjects are preliminary and should be interpreted with caution.