Observations are made of the radial evolution of various solar wind bulk parameters between 1 and 10 AU by using data from Voyagers 1 and 2 and the IMP 8 spacecraft. The solar wind bulk velocity, the product of the mass flux and the square of the heliocentric distance, and the product of density and the square of the heliocentric distance are all found not to vary with heliocentric distance. The proton temperature varies with heliocentric distance as R0.7¿0.2. At 1 AU, the density is anticorrelated with velocity. This anticorrelation vanishes at larger distances from the sun. The solar wind bulk velocity and thermal speed are correlated over the entire region between 1 and 10 AU. An attempt is made to explain the radial variation of the solar wind proton temperature between 1 and 10 AU, and the global energy balance in the solar wind. Heating by thermal processes, conversion of bulk kinetic energy to thermal energy, and electron heat conduction are considered. It is found that the radial variation of the solar wind proton temperature between 1 and 10 AU can be explained by the addition of a heat flux due to heat conduction. This heat flux is comparable to the proton and electron thermal energy fluxes but decreases faster with heliocentric distances, varying as R-0.44.