Symmetries of the nonlinear optical properties (rotation, reflection, inversion) of a two-level molecular system coupled to the thermal reservoir and interacting with classical electromagnetic fields, are studied. The thermal reservoir induces a time-dependent shift of the two-level resonance frequency. A perturbative methodology based in cumulant expansions to obtain the nonlinear macroscopic polarization, is employed. These symmetry properties show the dependence of the measured of the nonlinear optical properties with the variations in the frequencies of the incident fields.