In situmagnetoresistance measurements on lithographically defined spin-valve elements were performed by means of Lorentz transmission electron microscopy. The observation of a magnetic domain structure and the simultaneous magnetoresistance measurement by applying controlled field and controlled current have led to a clear correlation between giant magnetoresistance and changes in the magnetic domain structure. A study of the spin-valve behavior with the increase of the applied current value is also shown. ©1997 American Institute of Physics.