By means of the MINDO/3 method the potential energy hypersurfaces for the 1,3-dipolar cycloadditions of carbonyl and azomethine ylides to ethylene have been studied. In the first place, the synchronous formation of the two new sigma bonds has been considered. In the second place, the asynchronous region of the surface has been explored. The asynchronous mechanism appears to be more favourable, there being two transition states and an intermediate. Finally, the physical meaning of the results obtained is discussed. © 1979.