The basic nature of the transport and dispersion of a dense gas plume in the simulated atmospheric boundary layer of a wind tunnel was investigated both in flat terrain and over a ramp. Measurements were made of the concentration fields downstream of ground-level, circular sources. These measurements consisted of longitudinal ground-level, vertical, and crosswind profiles at various distances downwind. Both neutrally buoyant (air) and negatively buoyant (CO2) source gases were used so that the specific effects of the density difference could be observed. Similarly, measurements were made in both flat terrain and over the ramp (14 degree slope followed by an elevated plateau) so that specific effects of the terrain could be observed. The resulting dense plume was significantly wider in the lateral direction and much narrower in the vertical direction, yet the longitudinal ground-level concentration profile downwind was essentially identical to that from the neutral plume. The lateral profiles of the neutral plumes were essentially Gaussian in character, whereas the dense gas plumes exhibited top-hat distributions. The net effect of the ramp on the dense gas plume was a small reduction in ground-level concentration, which was quite similar to that of the neutral plume. (Copyright (c) 1988 Elsevier Science Publishers B.V.)