1. The 3mV depolarization of pig lymphocytes observed within 2 1/2 min of treatment with concanavalin A [Felber & Brand (1983) Biochem. J. 210, 885-891] is dependent on the presence of high extracellular [Na+]. 2. The concanavalin A-induced changes in membrane potential at high and low extracellular [Na+] are quantitatively explained by an increase in the electrogenic permeability coefficient for Na+ (PNa). This rises from a negligible value in resting cells to around 4% of the permeability coefficient for K+ or Cl- in stimulated cells. 3. Concanavalin A induces a 4mM increase in the Na+ content of pig lymphocytes. This increase in intracellular [Na+] is not due solely to stimulation of electrogenic Na+ influx resulting from the rise in PNa. 4. Thus concanavalin A stimulates both an electrogenic pathway for Na+ influx, resulting in a small depolarization of the plasma membrane, and a non-electrogenic Na+ influx pathway, resulting in a rise in intracellular [Na+].