We firstly examine the possible roles of mass loss and mixing as a function of stellar masses. Then we review the present observational status about additional mixing processes in massive stars. Various evidences from main-sequence OB stars, from blue supergiants, from SN 1987A and from Wolf-Rayet stars are given in favour of a significant mixing in radiative zones of rapidly rotating massive stars. We also particularly examine the role of mass loss on the ratio Δ Y/Δ Z of the relative helium-to-metal enrichments. We show that mass loss is far from sufficient to explain the high observed δ Y/ Δ Z ratios. We suggest that swallowing of heavy elements in black holes occurs. In order to account for the observed Δ Y/Δ Z ratios in low Z HII regions, it is necessary that black holes form above initial masses of about 20 {ie5-1}.