The present communication reports the effects of N-nitroso-pyrrolidine (NPYR) on some essential metabolic pathways in isolated hepatocytes. Isolated cells prepared by the collagenase-perfusion technique are incubated for 4 hours in suspension in a Waymouth medium, either in the absence or in the presence of NPYR (20 to 40 mmol/l). Under these conditions, NPYR, even at 40 mmol/l, has no effect on the vital staining of the cells by erythrosin B and does not increase the leakage of LDH, indicating no lethal effect. However, the glycogen synthesis which occurs in control cells is inhibited in presence of NPYR and the intracellular glycogen is even degraded. This effect is accompanied by reactivation of phosphorylase 'a'. NPYR also affects the protein synthesis capacity of the isolated hepatocytes. Incubation in the presence of non-metabolizable labelled amino acids has demonstrated that such an effect could be explained by a specific action of NPYR on the L-transport system for amino acids through the hepatic plasma membrane.