Increase in moisture content or ambient temperature up to about 165° C, increases the maximum extension obtained by drawing nylon 66 filaments and the resulting orientation of the molecules due to their increased mobility. Above 160° C disorientation due to thermal energy offsets this, and lower birefringences were observed for a given extension. The results are consistent with adiabatic drawing at a neck up to a natural draw ratio determined by the ambient temperature, combined with isothermal drawing uniform along the length of the specimen.