Mantle flow occurs through both linear and nonlinear creep mechanisms and the style of convection is strongly influenced by the dominant creep mechanism in a composite rheology. We show here by numerical modelling of convection with a composite flow law that the nonlinear, strain-rate dependence of mantle rheology would dominate over the linear (Newtonian) portion for sufficiently vigorous convection such as might have prevailed in the early Earth. This rheological transition occurs at a Rayleigh number value which increases dramatically with higher values of the activation energy. For activation energy values for dislocation and diffusion creep of about 100 and 70 kJ/mole respectively we find a transition between Rayleigh numbers 106 and 107. Corresponding values of the transition-stress agree with estimates for the lower mantle from empirical data for perovskite analogue. These results suggest that early in the Earth's thermal history a transition would have taken place from a primarily non-Newtonian flow to a Newtonian, temperature-dependent style of mantle convection. With a high activation energy, greater than 125 kJ/mole, the mantle would have been in a Newtonian regime throughout most of its thermal history. ¿ American Geophysical Union 1995