We propose a simple mathematical model of liver S-adenosylmethionine (AdoMet) metabolism. Analysis of the model has shown that AdoMet metabolism can operate under two di!erent modes. The "rst, with low metabolic rate and low AdoMet concentration, serves predominantly to supply the cell with AdoMet, the substrate for various cellular methylation reactions. The second, with high metabolic rate and high AdoMet concentration, provides an avenue for cleavage of excess methionine and can serve as a source of cysteine when its increased synthesis is necessary. The switch that triggers interconversion between the &&low'' and &&high'' modes is methionine concentration. Under a certain set of parameters both modes may coexist. This behavior results from the kinetic properties of (i) the two isoenzymes of AdoMet synthetase, MATI and MATIII, that catalyse AdoMet production; one is inhibited by AdoMet, whereas the other is activated by it, and (ii) glycine-N-methyltransferase that displays highly cooperative kinetics that is di!erent from that of other AdoMet-dependent methyltransferases. Thus, the model provides an explanation for how di!erent cellular needs are met by regulation of this pathway. The model also correctly identi"es a critical role for glycine N-methyltransferase in depleting excess methionine in the high mode, thus avoiding the toxicity associated with elevated levels of this essential amino acid.