Excitation of the fine structure levels of the ground state of atomic oxygen, O (3P), is currently believed to be the dominant cooling mechanism for the electron gas of the bottomside ionosphere (roughly, below 250 km). Recent refinements in theoretical excitation cross section calculations have reduced its magnitude by about a factor of 3. This cooling rate, however, has never been measured. The work reported here presents the results of the first experiment designed to measure this cooling rate. The technique is to measure the time relaxation rate of the temperature of the lower F region electron gas, after it is artifically heated by radio waves from a ground-based high-power HF transmitter. The experimental measurements presented favor the cooling rate corresponding to the lowest of the currently available theoretical excitation cross sections. The measurements do not, however, exclude the possibility that even this rate might still be too high by a considerable factor.