We study the complete merger of two massive stars inside a common envelope and the subsequent evolution of the merger product, a rapidly rotating massive supergiant. Three qualitatively different types of mergers have been identified and investigated in detail, and the post-merger evolution has been followed to the immediate presupernova stage. The ``quiet merger case does not lead to significant changes in composition, and the star remains a red supergiant. In the case of a ``moderate merger, the star may become a blue supergiant and end its evolution as a blue supergiant, depending on the core to total mass ratio (as may be appropriate for the progenitor of SN 1987A). In the case of the most effective ``explosive merger, the merger product stays a red giant. In last two cases, the He abundance in the envelope is increased drastically, but significant s-processing is mainly expected in the ``explosive merger case.
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