for the spectrophotometric determination of xylulose 5-phosphate, ribulose 5-phosphate, and combined ribose 5-phosphate and sedoheptulose 7-phosphate in tissue extract is presented. Using commercially available enzymes all three assays come to a clear endpoint with the assays described. Values for these metabolites in liver in three dietary states are reported; 48 h starved, ad libitum feeding of standard NIH rat ration, and meal feeding of a fat-free diet. Xylulose 5-phosphate values were 3.8 + 0.3, 8.6 + 0.3, and 66.3 + 8.3 nmol/g. Ribulose 5-phosphate values were 3.4 + 0.3,5.8 k 0.2, and 37.1 + 5.3 nmol/g. Combined ribose 5-phosphate and sedoheptulose 'I-phosphate were 29.3 + 0.3, 38.2 f 1.2, and 108.2 k 14.5 nmol/g. The ratio of measured tissue content of [xylulose 5-phosphate]/[ribulose 5-phosphate] was found to be 1. I2 + 0.07 in starved animals, 1.48 f 0.04 in ad libitum fed animals and 1.78 + 0.03 in low-fat meal fed animals. These data are in good agreement with the range of equilibrium constants reported for this reaction, suggesting that the ribulose 5-phosphate 3-epimerase reaction (EC 5.1.3.1) is a near equilibrium reaction despite a more than 1 O-fold change in the tissue content of these metabolites.