The transient character of the jet issuing from an upward nozzle centered in the bottom of a vertical cylindrical tank into bulk liquid of a different density was measured for varying jet Reynolds numbers and ratios of fluid densities using flow visualization and PIV. Positively buoyant jets penetrate to the free surface, driven by both momentum and buoyancy in the upward direction. The lighter jet fluid stratifies in a layer above the bulk liquid. Upon starting, a negatively buoyant jet has three stages. First the jet penetrates to its maximum height in the tank. Then the jet penetration decreases due to the downward backflow of heavier fluid surrounding the jet, which reduces the jet's upward momentum. Finally the jet penetration height fluctuates around a mean value about 70% the maximum height of penetration. For small negative densimetric Froude numbers (-6075≤Fr≤-250), the flow is fountain-like. The downward flow turns radially outward as it reaches the bottom of the tank and eventually an annular recirculation zone forms at the bottom of the tank with vortical motion opposite the vorticity of the inflowing jet. For large negative Froude numbers (Fr≤-7770), the spreading of the jet extends far enough so the annular downward flow is along the walls of the tank resulting in a large annular recirculation zone. Scaling the jet penetration and time with the buoyancy flux and the jet momentum flux results in collapse of the transient jet penetration over a wide range of Froude numbers.