The lower ionosphere has been heated in a controlled experiment at the Arecibo ionospheric Observatory. Deposition of energy from a 40 Mhz transmitter has approximately doubled the ambient electron temperature at an altitude of 80 km. Energy from the radio wave is deposited in the ionosphere as a result of collisions of the electrons with the neutral molecules. Cross-modulation experiments (performed elsewhere) have gained information on electron number density and collision frequency between 60 and 90 km. The experiment described here will possibly allow determination of the electron collision frequency and thermal relaxation times between 70 and 120 km, and may increase the understanding of the interaction of plasmas and radio waves. The heating is measured with the 430 Mhz incoherent backscatter radar system. The temperature dependence of the electron backscatter cross-section enables the change in electron temperature to be inferred from backscatter power measurements. Heating with megawatt pulses of 1, 2 and 5 ms duration confirmed predictions of the magnitude of heating.