1. Phosphoenolpyruvate carboxykinase was assayed by three methods: (i) incorporation of H14CO3- into oxaloacetate: (ii) conversion of oxaloacetate into phosphoenolpyruvate, subsequently assayed enzymically; and (iii) transfer of 32P from [γ-32P]GTP to oxaloacetate. 2. Enzyme activity is increased in liver and epididymal adipose tissue in alloxan-diabetes and starvation, and in kidney in starved, acidotic and steroid-treated animals. 3. The ratios of the ‘back’ to the ‘forward’ reactions in liver, kidney and epididymal adipose tissue are different and characteristic of each tissue; they differ markedly from values reported for the purified mitochondrial enzyme. 4. The ratio of the ‘back’ to ‘forward’ reaction in any one tissue is constant in adrenalectomized, diabetic, acidotic and steroid-treated animals. 5. In starved animals, the ratio is increased in liver and kidney, but decreased in epididymal adipose tissue. 6. Administration of l-tryptophan results in an acute (1h) increase in activity measured in the ‘forward’ direction alone in liver and epididymal adipose tissue, but not in kidney.
