A novel laser interferometric technique involving the use of an unstable external resonator is presented. The unstable resonator greatly facilitates electron density measurements in the presence of plasma density gradients transverse to the direction of beam propagation. The technique is used to obtain time‐resolved radial electron density distributions in the implosion stage of a linearz‐pinch discharge in argon at filling pressures of 0.1 and 1.0 Torr.