An homeotropic cell is built with a thin layer (5–25 &mgr;m) of nematic liquid crystal such as MBBA sandwiched between two indium oxide coated glass plates. The transient behavior of the cooperative angular variation of molecules submitted to an electric field step is studied theoretically. Experimental results are obtained by optical detection of the birefringence variations in the cell. Comparison of these studies allows one to determine the viscosity coefficient &ggr;1(−&lgr;1in Leslie's notation).