The EPA Meteorological Wind Tunnel was used to simulate a rural atmospheric boundary layer. Experiments were conducted on flow and dispersion over three-dimensional, conical hills of two different slopes (26.5 degrees and 17.5 degrees) whose heights were 1/3 of the boundary layer depth. These measurements were made both in undisturbed and disturbed (in presence of hill) flows, using hot-wire anemometers and hydrocarbon analyzers. Enhanced vertical and lateral dispersion were evident in the immediate wake region of the hill. This phenomenon reflects the increased turbulence levels in the wake. The results of this study clearly illustrate that the slope of a conical hill is a most important parameter; it determines the location of flow separation, the size of the cavity region, and the characteristics of the wake flow dispersion.