Intraperitoneal injection of 5 mumol of acetone/g, body weight, into 3 rats previously fed 1% acetone (v/v) in their drinking water resulted in the appearance in blood serum of 16 +/- 2 nmol of 1,2-propanediol/ml and 8 +/- 1 nmol of 2,3-butanediol/ml. No detectable 1,2-propanediol or 2,3-butanediol was found in the serum of animals after acetone or saline injection without prior addition of acetone to drinking water or in the serum of animals injected with saline after having been maintained on drinking water containing 1% acetone. These data suggest that acetone both acts to induce a critical enzyme or enzymes and serves as a precursor for the production of 1,2-propanediol. It is also clear from these data that chronic acetone feeding plays a role in 2,3-butanediol production in the rat. Microsomes isolated from the liver of animals maintained on drinking water supplemented with 1% acetone contained two previously unreported enzymatic activities, acetone monooxygenase which converts acetone to acetol and acetol monooxygenase which converts acetol to methylglyoxal. Both activities require O2 and NADPH. Prior treatment with acetone increased serum D-lactate from 9 nmol/ml +/- 9 nmol/ml in control animals to 77 +/- 36 nmol/ml in acetone-fed animals after injection with 5 mumol of acetone/g, body weight. This is consistent with methylglyoxal being a by-product of acetone metabolism. Two pathways for the conversion of acetone to glucose are proposed, the methylglyoxal and the propanediol pathways. The methylglyoxal pathway is responsible for the conversion of acetone to acetol, acetol to methylglyoxal, and the subsequent conversion of methylglyoxal to glucose. The propanediol pathway involves the conversion of acetol to L-1,2-propanediol by an as yet unknown process. L-1,2-Propanediol is converted to L-lactaldehyde by alcohol dehydrogenase, and L-lactaldehyde is converted to L-lactic acid by aldehyde dehydrogenase. Expression of these metabolic pathways in rat appears to be dependent on the induction of acetone monooxygenase and acetol monooxygenase by acetone.