The thermal response of the nighttime F region ionosphere to local heating by HF radio waves has been observed with the incoherent scatter radar at Arecibo, Puerto Rico. The observations consist of high-resolution space and time variation of the electron temperature as a high-power HF transmitter is switched on and off with a period 240 s. As soon as the HF transmitter is turned on, the electron temperature begins to rise rapidly in a narrow altitude region near 300 km, below the F2 layer peak. The electron temperature perturbation subsequently spreads over a broader altitude region. The observations are compared with the anticipated thermal response of the ionosphere based on numerical solutions of the coupled time-dependent heat conduction equations for the electron and composite ion gases and are found to be in good agreement over the entire altitude region covered by the observations. The calculations show that heat conduction is responsible for the spreading of the electron temperature enhancement outside the region where the radio wave energy is deposited. They also show that a smaller, but experimentally observable, ion temperature enhancement takes place while electron temperatures are enhanced by 40% or more of their equilibrium values. For the data presented they also show ionospheric absorption of roughly half the radiated HF energy, with anomalous absorption roughly equal to deviation absorption heating.