Night-time ionospheric plasma density profiles over the equatorial station Natal, Brazil, measured simultaneously using a rocket-borne Langmuir Probe (LP) and a High Frequency Capacitance Probe (HFC) on two occasions, show considerable differences, especially above the F-region peak. In this height region, the electron density values estimated from the LP ion currents are considerably less than those estimated from the HFC data, while below the F-region peak, the LP electron densities seem to be slightly greater than the HFC electron densities. From considerations of the basic principles of the two experiments it is seen that while the higher LP electron densities at lower heights are probably due to an increase in the effective sensor surface area caused by the sheath formation, in the region above the F-peak the relative lower plasma density obtained from LP current seems to be caused by (a) a change in the floating potential of the rocket, (b) a decrease in the thickness of the plasma sheath, and (c) an increase in the mean ion thermal velocity relative to the rocket velocity.