Resonant enhancement of nonlinear susceptibility in sodium vapour resulting in an intensity dependent nonlinear refractive index was investigated using the Z-scan technique. Estimates for the dipole moment btained from the values of the near-resonant optical nonlinearity measured by us, in the vicinity of the D₁and D₂ sodium resonance lines, were found to be in good quantitative agreement with the reported values. The functional dependence of the observed nonlinear susceptibility on the extent of detuning of the laser frequency with respect to the resonant atomic transition was in agreement with the adiabatic following model.