Laboratory experiments are used to examine the effects of large amounts of internal heat generation on Rayleigh-B¿nard convection planforms in a constant viscosity, high Prandtl number fluid. Internal heat generation is simulated by lowerinig the boundary temperatures at a constant rate. The Rayleigh number (RaT) based on the imposed temperature difference is 1.5¿105 and the Rayleigh number based on the amount of internal heat generation (RaH) is varied from 0 to 3.0¿106. In the absence of internal heat generation the convection planforms are the symmetric spoke pattern in which connected spokes of ascending and descending flow have, on average, equal strength. With internal heat production, equal strength. With internal heat production, the planform is asymmetric with buoyancy concentrated in isolated descending arcuate trenches and circular plumes. The upwellings are nonbuoyant, although their planform is more connected than downwellings. The amount of time dependence is observed to increase with internal heat generation, and occurs by branching and propagation of the negatively buoyant trenches. ¿ American Geophysical Union 1990