The behavior of single drops of isotropic hydroxypropylcellulose solutions immersed in a polydimethylsiloxane matrix subjected to a constant shear rate was investigated using a rheo-optical technique. Our main results concern the nonstationary deformation and breakup mechanisms of drops characterized by large capillary numbers. The deformation of threads follows a pseudoaffine deformation for Ca/Ca-crit larger than 2.5 and above a certain strain. The end pinching mechanism occurs at a specific scaled strain, which includes the initial drop size and the applied shear rate. Capillary instabilities develop when the thread diameter reaches a critical value, d(r), inversely proportional to the applied shear rate and independent of the initial drop diameter. The time necessary for the total rupture of a thread depends on the initial drop diameter, the applied shear rate, and the critical thread diameter d(r). The droplet size resulting from the final rupture of a thread is found to be half that of the critical size, below which a drop deforms into a stable ellipsoid.