The chain-intrinsic fluorescence technique was used to investigate the development of orientation in the noncrystalline phase of PET film during drawing at 90 degrees C. It was confirmed that orientation develops faster at higher strain rate. In the low draw ratio (precrystallization) regime, the orientation-strain data at high strain rate can be fitted to the affine network model, but at low strain rate, where relaxation effects are substantial, this model is not applicable. We found considerable deviation from the linear stress-optical law : it appears that a stress threshold must be reached before significant orientation takes place and that the threshold value increases with strain rate. Crystallization onset seems to increase the rate of development of orientation at the lower strain rate, probably by providing additional junction points. At both high and low strain rates, however, there eventually occurs an abrupt decrease in the rate of development of orientation (and crystallinity) which coincides with a sharp increase in stress. By comparing our data to some results in the literature on uncrystallizable PEMT, we infer that the slowing of orientation development arises from chain slippage and that the increase in stress largely results from the reinforcing effect of crystallites and an increase in polymer viscosity arising from the interconnection of crystallites.