On the basis of transport theories appropriate to a radially expanding solar wind, new results for the evolution of the energy density in solar wind fluctuations at MHD scales are derived. The models, which represent a departure from the well-known WKB description, include the effects of ''mixing'', driving by stream-stream interactions (compression and shear) and interstellar pick-up ions as well as non-isotropic MHD turbulence. Magnetometer data from Voyager 1 and 2 and Pioneer 11 are compared to the turbulence-based models and close agreement is found between theory and data for a reasonable choice of parameters. ¿ American Geophysical Union 1996