Evolution of the N/O surface abundance ratio as a function of the mass-loss rate for a 32 M_⊙ star of solar metallicity and for initial rotation rates Ω / Ω_crit from 0 to 0.4, using the models of Ekström et al. (2012). The dashed lines correspond to the calculated value of N/O for the nebula M1-67, with its errors, and to the lower limit of the mass-loss rate. The thicker lines emphasize the part of the tracks compatible with the measurements. For clarity, the tracks are stopped at the beginning of the blue loop (data point No. 210 in Ekström et al. 2012).