The effect of hydrostatic pressure on the swelling of temperature-sensitive gels was studied by measuring the volume change of the beads of poly(N-isopropylacrylamide) gel, poly(N-n-propylacrylamide) gel, and poly(N,N-diethylacrylamide) gel under pressure up to 120 atm. The excess enthalpy and the excess volume of the gel-water systems during the volume phase-transition of the gels were measured. These measurements are helpful in understanding that the pressure sensitivity of these gels originates from their temperature sensitivity and is the consequence of the increase of their LCST with pressure. An equation relating the LCST of the temperature-sensitive gel to the pressure was obtained through thermodynamical analysis. These efforts contribute to reveal the interdependence between the pressure sensitivity and the temperature sensitivity of hydrogel.