The S(2.3 GHz) and X band (8.4 GHz) tracking links with the Viking orbiters have been used to study the atmosphere and topography of Mars at latitudes ranging from 74¿S to 73¿N. Data acquired in the troposphere show large meteorological changes with near-surface temperatures ranging from 150¿ to 250¿K. Inversion layers were observed above the polar caps and in areas engulfed by dust storms. At other locations the temperature was found to decrease with increasing height at a rate equal to the dry adiabatic lapse rate. Seasonal pressure variations, presumably caused by changes in the polar frost deposits, were observed. At the 5-km altitude level the atmospheric pressure ranged from about 3.5 to 4.8 mbar during the Martian year. The measurements in the upper atmosphere yielded double- and single-layered electron density profiles on the sunlit and dark sides of the planet, respectively. A comparison of the Vi-scale height of the ionosphere appear to be functions of solar activity. The topographic occulation data agree well with the elevation contours that are shown on U. S Geological Survey map M 25M 3 RMC except in a few areas such as the south polar regionand the Alba Patera region of Mars. In the south polar region the occultation measurements yielded surface elevations ranging from 3 to 6 km relative to the reference gravity equipotential surface or reference areoid. The terrain around Alba Patera was found to be 6 to 7 km high.