Methods and apparatus are provided for intravascularly-induced neuromodulation using a pulsed electric field, e.g., to effectuate irreversible electroporation or electrofusion, necrosis and/or inducement of apoptosis, alteration of gene expression, changes in cytokine upregulation, etc., in target neural fibers. In some embodiments, the intravascular PEF system comprises a catheter having a pair of bipolar electrodes for delivering the PEF, with a first electrode positioned on a first side of an impedance-altering element and a second electrode positioned on an opposing side of the impedance-altering element. A length of the electrodes, as well as a separation distance between the first and second electrodes, may be specified such that, with the impedance-altering element deployed in a manner that locally increases impedance within a patient's vessel, e.g., with the impedance-altering element deployed into contact with the vessel wall at a treatment site within the patient's vasculature, a magnitude of applied voltage delivered across the bipolar electrodes necessary to achieve desired neuromodulation is reduced relative to an intravascular PEF system having similarly spaced electrodes but no (or an undeployed) impedance-altering element. In a preferred embodiment, the impedance-altering element comprises an inflatable balloon configured to locally increase impedance within a patient's vasculature. The methods and apparatus of the present invention may be used to modulate a neural fiber that contributes to renal function.