Measurements were made in mercury, for turbulent flow and constant flux heating in a vertical pipe, in order to determine the extent to which the velocity and temperature distributions are affected by buoyancy forces. With increasing heat flux, velocity profiles at Reynolds numbers of 20,000 to 60,000 were found to be markedly distorted in comparison with the isothermal velocity profile. Even very low heat input caused significant distortion, while at high heat input a limiting profile shape was approached, with the center velocity well below the mean and the maximum occurring in the vicinity of the wall. Eddy diffusivities of heat and momentum calculated from the measured profiles exhibit a considerable variation with heat input, indicating that buoyancy forces not only change the radial shear stress distribution but also alter the nature of the turbulence in the pipe.