Poly(amino acid) in a n intermediate state of its helix-coil transition is known t o be in a hinged rodlike conformation. In this work, the responses of poly(amino acids) in the hinged rodlike conformation against a n elongational flow field were investigated by monitoring their flow-induced birefringence. Poly(L-glutamic acids) (PGA) and poly(y-benzyl-L-glutamate) (PBLG) were examined as polyelectrolyte and noncharged poly(amino acids), respectively, and the results were compared. In the plots of flow-induced birefringence, An, against strain rate, E, for hinged rodlike PBLG, there was a critical strain rate, Eo, below which An was not observed. Over Co, the birefringence pattern observed was identical with t h a t of rodlike molecules. T h e An vs. C plot for hinged rodlike PGA had characteristics of a rigid rod a t any strain rate and there was no Eo observed. T h e rotational diffusion coefficient, D,, of PBLG in the hinged rodlike conformation was larger than that for its helical conformation, while D, for the hinged-rodlike PGA was smaller than that for its helical conformation. It is concluded that the hinged-rodlike PGA molecule is in a n 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 Co hinged rodlike PBLG molecules collapse t o a conformation optically anisotropic and mechanically rigid.