Various molecular parameters characterizing the solution properties of poly(N-n-propylacrylamide) (PNnPAM) in water were determined over the molecular weight range of(13.3 to 159) x 10(4) at 10 degrees C by use light scattering. and viscosity measurements. PNnPAM molecules behave as highly hydrated flexible coils. The Laplace inversion method of autocorrelation unction of the scattered light intensity ascertained the molecular weight dependence of molecular parameters. Thermal response of PNnPAM molecules in water was studied concerning that the PNnPAM gel exhibits a unique and sharp volume phase transition in contrast to poly(N-isopropylacrylamide) gel. Drastic decrease of the second virial coefficient was observed in a very narrow temperature region near the theta temperature, which was determined to be 22.54 +/- 0.01 degrees C. Thermal response of PNnPAM molecules psas discussed relating to the excluded volume effect, the hydrophobic interaction, and the chain stiffness.