Terpolymers composed of N-n-propylacrylamide (NPAAm), butyl methacrylate (BMA), and N,N-diethylaminaethyl methacrylate (DEAEMA) were prepared in an attempt to investigate the temperature-induced phase transition and its mechanism. Poly(NPAAm) showed the lower critical solution temperature (LCST) around 24 degrees C in water. With the incorporation of DEAEMA with NPAAm, the LCST change was characterized by an initial increase. However, the LCST was shifted to the lower temperature at the later stage. This might be explained in terms of hydrophilic/hydrophobic contribution of DEAEMA to the LCST. The swelling behavior of copolymer gel in the various solvents and spin-lattice relaxation time (T-1,) study by NMR strongly suggested the hydrophilic/hydrophobic contribution of DEAEMA to the LCST depending on the local environment.