The present invention is directed to the upgrading of heavy hydrocarbon feedstock that utilizes a short residence pyrolytic reactor operating under conditions that cracks and chemically upgrades the feedstock. The process of the present invention provides for the preparation of a partially upgraded feedstock exhibiting reduced viscosity and increased API gravity. This process selectively removes metals, salts, water and nitrogen from the feedstock, while at the same time maximizes the yield of the liquid product, and minimizes coke and gas production. Furthermore, this process reduces the viscosity of the feedstock in order to permit pipeline transport, if desired, of the upgraded feedstock with little or no addition of diluents.