SYNOPSISThe quadratic nonlinear optical susceptibility of a solution of helical polymers and of samples with helices oriented parallel to each other is calculated for regions of characteristic vibrations and of their overtones. The important role of electrooptical and of mechanical anharmonicity for exhibition of overtones in nonlinear spectroscopy is shown. The possibility of the appearance of the giant polarizabilities near bound overtone st,ates is analyzed. The overtone spectrum of amide I is modeled numerically.