A mechanism for pumping energy from a biharmonic optical radiation field to the rotational excitation of a symmetric molecule is suggested. It is shown that a biharmonic field with difference frequency equal to four times the molecular rotational constant Be "triggers" the initial (J=1 to J=2) direct rotational transition and synthesizes an intramolecular field at the sequence of resonant frequencies Ωj =4Be(1+J/2), where J=1,2,3,.... Simultaneous forcing of the molecule by fields of these two types is accompanied by co-phased energy transfer from the biharmonic field to successively higher rotational states of the molecule. The threshold and limiting values of the rotational excitation are determined. The estimates made agree well with the published experimental data.