We present a study of the heliocentric distance, frequency, and stream structure dependence of the amplitudes of interplanetary fluctuations in the velocity and magnetic field from 0.3 to nearly 20 AU and for spacecraft-frame periods of 10 days to a few hours. The results constrain theories of the acceleration of the solar wind and of the evolution of fluctuation amplitudes. We show that the magnetic fluctuation amplitude normalized by the strength of the magnetic field is largely independent of stream structure at the scales studied. Using this fact, we find that the WKB prediction generally provides a good approximation to the magnetic amplitude evolution for all but the smallest scales in the inner heliosphere, which show a dissipation with respect to the WKB prediction, and the largest scales in the outer heliosphere, which decrease more slowly than the simple theory predicts. Velocity fluctuations are shown to generally follow the level of the magnetic fluctuations at the higher frequencies of this study, although it is the energy densities of the velocity and magnetic fluctuations that correlate well rather than the velocity and magnetic fluctuations themselves. Near 0.3 AU the energy in the largest scale velocity fluctuations is, on average, a few times larger than that in the magnetic fluctuations, but with increasing heliocentric distance the magnetic fluctuations become slightly dominant on all scales. The dominance of the kinetic energy in the inner heliosphere is limited to the radial component of the velocity, which reflects the large energy available in streams, while the transverse components exhibit a more ''wave-like'' near equipartition between kinetic and magnetic energy. The evolution found here is qualitatively consistent with the predictions of a recent theory of the turbulent evolution of the fluctuations. Also, the observed agreement with WKB theory at intermediate to large scales in the inner heliosphere suggests that the power level of waves near the Sun is insufficient to accelerate high-speed streams. ¿ American Geophysical Union 1990