Thanks.
Sorry, I forgot to mention that it was midi. I'm trying to transpose several measures an octave down at once and not note by note. I haven't seen anything possible using Notation or Piano roll: when I select several notes, nothing happens. I have tried « Shift », « Option «, « Command » keys and so on but no result.
Moreover, I have tried the menu Transpose: you can select a number of measures to transpose from one key to another but it is not possible to transpose an octave down (or up). So…