Swelling and stress-relaxation under tension for poly(N-isopropylacrylamide) (PNIPA) hydrogels have been investigated in water and in liquid paraffin at various temperatures. Initial Young's modulus is much enhanced due to the change from the swollen to collapsed state, but no difference has been observed in the initial and osmotic Poisson ratios for the gels not only in the collapsed but also in the swollen states. The enhancement of modulus originates from the introduction of physical crosslinks in the collapsed state. In liquid paraffin, the gels in the collapsed phase show large degree of stress reduction in the course of stress-relaxation due to the breakdown of the physical crosslinks. The stress-relaxation induced by swelling is observed for both phases in water, but the degree of stress reduction is larger in the collapsed phase than in the swollen phase. A comparison between experimental results and theory applicable to swelling and swelling-induced stress-relaxation is made to show that the agreement is not so good. For the stress-relaxation of the collapsed gel in water, it is suggested that the swelling couples with the breakdown of physical crosslinks at short times, and the trance of the coupling is observed even at long times.