The guest-host interactions of pleochroic dyes dissolved in liquid crystals are frequently used to obtain information about the structure (usually order parameter) of the mesophases1. Almost no attention has been paid, however, to the possible influence of dipolar or highly polarizable dye molecules on the liquid crystal solvent, mainly because the solubility of the dyes is very low and no significant macroscopic effect is expected in dilute solution (usually 0.1-1%). The synthesis of liquid crystal copolymers containing dye and mesogenic units2 opens a way to incorporate a dye in amounts large enough to change the structure of the mesomorphic system. Here we report structural transformations occurring in side chain liquid crystal copolymers containing thermochromic spiropyran side groups. The spiropyran to merocyanine dye conversion occurs on heating of the copolymers. Physical crosslinking of the macromolecules due to aggregation of the dye moieties gives rise to a network formation, responsible for the appearance of a new rheo-optical effect, observed above the clearing point.