This paper reports studies of vesicles made by the extrusion of lipid suspensions through the pores of polycarbonate membranes as a function of the concentration of the lipid suspension and of the average pore size of the membranes. Experiments clearly show that the applied pressure, rattler than the flow rate achieved, determines the size of vesicle produced. Vesicle size decreases as the applied pressure increases for all pore sizes measured. However, the size of vesicles produced in larger pores reduces more quickly as the pressure is increased as compared to those produced in smaller pores. While the size of vesicle produced and the polydispersity of the vesicle population is only weakly dependent on the concentration of the lipid suspension, the minimum pressure required for extrusion increases dramatically for higher lipid concentrations. A method for estimating lysis tension of the membrane using the minimum extrusion pressure is discussed; for POPC (1-palmitoyl-2-oleoyl-sn-phosphatidylcholine) we measure a lysis tension of 7.7 +/- 0.3 mN/m.