Alternate scan 13C and 31P NMR has been used to follow the metabolism of 13C-labeled substrates, in the presence and absence of insulin, in isolated perfused liver from fasted rats. Because both 31P and 13C NMR spectra are recorded almost simultaneously with this method, both phosphate metabolites and 13C-labeled metabolites are measured, noninvasively and repetitively, to give an immediate, broad survey of the hepatic response to a variety of stimuli. During the metabolism of [2-13C]pyruvate, [1,2-13C]ethanol, and NH4+, 13C-labeled glycogen increases synchronously with, and at the same rate as, the synthesis of 13C-labeled glucose; thus, glycogenesis was essentially a gluconeogenic process under our conditions and was unaltered by the presence of insulin. From the position of the 13C-labeled citrate peak observed in liver, the measurement of KD for the citrate-magnesium complex under our conditions, and the expression relating these quantities to the concentration of free Mg2+, the intracellular level of free Mg2+ is estimated to be 0.46 +/- 0.05 mM. Later administration of glucagon led to a rapid decrease in glycogen and citrate and a 44% increase in glycero-3-phosphocholine (GPC); increase in GPC is consistent with stimulation of liver phospholipase activity by glucagon. Simultaneous administration of two different 13C-labeled substrates, or one doubly labeled substrate, introduced multiplet structure arising from spin-spin interaction between labeled adjacent carbons into the peaks of several key metabolites. The 13C NMR intensity distributions within the several multiplets are used, within the context of a first-order model for fluxes into the Krebs cycle, to estimate relative fluxes under the conditions of the experiment.