Erythrocytes of lamprey Lampetra fluviatilis were incubated in standard isotonic medium at 20°C with 22Na to determine the unidirectional Na+ influx. Cell incubation in the presence of various protein phosphatase inhibitors (NaF, cantharidin, calyculin A) led to a considerable increase of Na+ transport into erythrocytes. The stimulation of Na+ influx into erythrocytes rose with increase of concentration of calyculin A within the range of 10–100 nM. The calyculin A concentration producing a 50% activation of Na+ transport amounted to 41.5 nM. Under optimal experimental conditions, the Na+ influx increased from control level of 5–8 to 20–40 mmol/l cells/h under effect of protein phosphatase blockers. The Na+ transport induced by these inhibitors was completely suppressed on addition of amiloride to the incubation medium. The treatment of lamprey erythrocytes with protein phosphatase inhibitors was accompanied by a small (∼12%), but statistically significant decrease of intracellular Na+ content. A small decrease of intracellular K+ content in erythrocyte was observed only under the effect of NaF. The obtained data allow making the conclusion that protein phosphatases of the PP1 and PP2A types play a significant role in regulation of Na+ transport across the lamprey erythrocyte membrane in both directions.