H-1 and C-13 n.m.r. spectra of poly(N-isopropyl acrylamide) (PNIPA) in D2O and CDCl3 were measured at several temperatures encompassing the lower critical solution temperature (LCST) about 32 degrees C of the PNIPA/water solution, using a high resolution n.m.r. spectrometer (500 MHz for proton). The PNIPA/CDCl3 is homogeneous in the temperature ranging from 16 to 36 degrees C and the hydrogen bonds of the amide groups are weakened by the solvation of CDCl3 and broken by heating. In the PNIPA/D2O solution, the proton and C-13 spectra are well resolved below the LCST. Whereas at temperatures above the LCST, all the resonance peaks except that for water proton became broad and the spectra lost their fine structure. The integral intensity of the water proton relative to that of the lone proton in the isopropyl group increases with increasing temperature, suggesting that some water molecules appear to be released out of the hydrated shells around the polymer chains. When the solution was heated from 22 to 35 degrees C, the spin-lattice relaxation time T-1 for the proton of the methyl group in the isopropyl residue increased, while that for the protons of the methylene and methyne groups of the backbone chain decreased. This indicates that during heating the relaxation of the isopropyl side chain slows down and that of the main chain speeds up. The phase separation was qualitatively interpreted with the Nemethy-Scheraga model for the hydrophobic bonding.