We describe an automated kinetic assay for acetoacetate in blood. Acetoacetate is enzymatically reduced to D-beta-hydroxybutyrate and the reaction is monitored for 60 s with a reaction-rate analyzer. This technique allows low concentrations of acetoacetate to be measured with good precision and overcomes many of the problems associated with other automated techniques. Our studies on the stability of acetoacetate emphasize the need for care in handling specimens. The use of a reaction-rate analyzer, an item of equipment common to most laboratories, allows for rapid handling of samples in small or large batches, depending on the needs of the laboratory.