[en] Using the equations of motion obtained from the Nambu Lagrangian, closed strings are studied. Interactions are introduced in the system via certain topological transformations, using the method of functional integration. The Born term turns out to be equal to the Virasoro-Shapiro amplitude. Not all the variables used in the calculation are independent and the physical states therefore satisfy certain conditions. However the interactions are such that those states not satisfying the extra conditions decouple from the rest. The amplitude for the scattering of excited states, the vertex functions and the volume elements for tree and one-loop amplitudes are calculated. The amplitude for tree or one-loop graphs can be simply obtained from the corresponding amplitude in open string theory. (Auth.)