The generation process of poly(N-isopropyl acrylamide) (PNIPA) gels has been studied by time-resolved light scattering. The gelation was initiated by adding a redox initiator and cross-linker (N,N＇-methylenebisacrylamide, BIS). The scattered intensity was observed at a fixed angle of 60 degrees as a function of polymerization/gelation time, t. The scattered intensity had an abrupt rise at t approximate to 20 min, then reached a plateau value having relatively large fluctuations. In the case of the corresponding PNIPA solutions, prepared without BIS, similar behaviour was observed, although the pulse height and the plateau intensities were significantly lower. In both cases, the abrupt intensity rise disappeared when the monomer concentration, C, was reduced to 88 mM or lower. In addition, a gel was not formed for C less than or equal to 88 mM even in the presence of BIS. It was concluded by viscometry and the monomer concentration dependence experiment that (i) the abrupt intensity rise corresponds to the gelation threshold and (ii) and so-called chain overlap concentration can be estimated by the appearance of the abrupt intensity rise tin this particular case, 88 less than or equal to C* less than or equal to 131 mM). The abrupt rise in the scattered intensity was also observed by small-angle neutron scattering, suggesting that the gelation threshold can be observed in a wide range of momentum transfer space.