The third order nonlinear optical susceptibilities χ⁽³⁾ of PbO-SiO₂ and PbO-B₂O₃ glasses have been measured by the third harmonic generation (THG) method using silica glass as reference. It is found that the third order nonlinear susceptibilities χ⁽³⁾ of each series of glasses increase with increasing lead content, corresponding to the increase in the polarizability per unit volume and the decrease in the energy gap of the glasses. The largest χ⁽³⁾ values obtained are 31.1×10^-14esu for a 66PbO⋅34SiO₂ glass and 31.2×10^-14esu for a 70PbO⋅30B₂O₃ glass, about 11 times larger than that of pure silica glass.