A low-input capacitance emissive probe heated by a CO2 laser has been used to investigate frequency-modulated electric potentials in a radio-frequency (rf) glow discharge plasma. An excellent time-response ability of our probe system to a sharp potential change was confirmed by potential measurements with applying a step function voltage to the plasma. The electric field distribution as the first derivative of the potential distribution obtained by the probe method was compared with that by the laser optogalvanic spectroscopy to ensure the reliability of the measurements in the sheath region. The probe method was applied to directly monitor a temporal variation of axial potential and electric field distributions between two parallel-plate electrodes in a rf argon plasma. The formation of ion sheath regions on both electrode surfaces was confirmed at any phase in a rf cycle. The results were consistent with a one-dimensional model for potential and electric field distributions in the sheath region. © 2003 American Institute of Physics.