The swelling behavior of poly(N-isopropylacrylamide) (PNIPA) gels in seawater at high pressures up to similar to40 MPa is examined in terms of human activity at deep sea. The neutral gel in seawater undergoes a continuous volume transition at 26degreesC at atmospheric pressure. Addition of the ionic group (sodium acrylate) does not have much effect on the swelling properties of the gels in seawater except that the transition temperature is somewhat increased. At high constant pressures up to similar to40 MPa, the gels undergo a continuous volume transition at 26-28degreesC as the temperature varies. Normally, the gel takes a swollen state at deep sea. The ratio of the volume change associated with the transition is more than 10, which is 3 times larger than that obtained by changing the pressure at a constant temperature. The results suggest a possibility that the phase transition of PNIPA gels is utilized for producing mechanochemical energy at deep sea. (C) 2005 Wiley Periodicals, Inc.