Helium (left panel) and nitrogen (right panel) surface mass fraction as a function of time for models of 60, 100, and 300  M_⊙ and rotation rates explained in the figure key. While for the fast rotators the enhancements are mostly due to rotational mixing, for the slowly rotating models the increase in helium and nitrogen in the 100  M_⊙ and 300  M_⊙ mass models is solely due to mass loss.