Poly(N-isopropylacrylamide-co-sodium acrylate) gels with N,N-methylene bisacrylamde (BIS) as crosslinker were prepared by free radical polymerization method at the temperature of 35 degrees C, which was just around the lower critical solution temperature (LSCT) of the hydrogels. The gels synthesized at 35 degrees C demonstrated strong swellability and fast responseability when compared with the gels synthesized at the temperature of 0 and 18 degrees C (below the LCST) and 50 and 80 degrees C (above the LSCT). The response rate and swelling behavior of poly(Nisopropylacrylamide-co-sodium acrylate) gels was investigated and characterized by the temperature-dependent swelling ratio and swelling and deswelling kinetics. The swelling behavior of the gels indicated that the synthesis temperature was the main factor when the swellability concerned and also had effect on the responseability of the resulting hydrogels. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112:123-128, 2009