Poly(amino acid) in an intermediate state of its helix-coil transition is known to be in a hinged rodlike conformation. In this work, the responses of poly(amino acids) in the hinged rodlike conformation against an elongational flow field were investigated by monitoring their flow-induced birefringence. Poly(L-glutamic acids) (PGA) and poly(gamma-benzyl-L-glu tamate) (PBLG) were examined as polyelectrolyte and noncharged poly(amino acids), respectively, and the results were compared. In the plots of flow-induced birefringence, Delta n, against strain rate, epsilon over dot, for hinged rodlike PBLG, there was a critical strain rate, epsilon over dot(0), below which an was not observed. Over epsilon over dot(0), the birefringence pattern observed was identical with that of rodlike molecules. The Delta n vs. epsilon over dot plot for hinged rodlike PGA had characteristics of a rigid rod at any strain rate and there was no epsilon over dot(0) observed. The rotational diffusion coefficient, D-r, of PBLG in the hinged rodlike conformation was larger than that for its helical conformation, while D-r for the hinged-rodlike PGA was smaller than that for its helical con formation. It is concluded that the hinged-rodlike PGA molecule is in an extended form and that the hinged-rodlike PBLG is hydrodynamically more compact and rigid than that in its quiescent state. It is deduced that at epsilon over dot(0) hinged rodlike PBLG molecules collapse to a conformation optically anisotropic and mechanically rigic.