Hi!
I am using a particle emitter set to grid mode. The way particles are emitted is fine, but I would like to invert the direction of the emitting. Right now the emission start from the left part of the object and continue to the right. I would like to invert this… Is there some way to do it?
Thanks in advance for any help! 
You could try going into edit mode, selecting all verts, and in the tools panel (T) clicking Normals: ‘Flip Direction’ button. Tab back into object mode and run the sim.
In object mode scale -1 on X axis (S X -1)
Then
In Edit mode scale all vertices -1 on X axis (S X -1)
Then
Recalculate normals (Ctrl+N)
thanks for the answers guys! In the end I made something between your two suggestions. rotated the object 180 on z in object mode and then scaled -1 in edit. I was able to do this because it is not a complex object (some kind of covex disk). It would be nice to have some more control over the grid emitter, but maybe it is possible to achieve this higher control using the texture emission, even if I remember I had some problem with this once I tried to use it.
Anyway, thank you again for your help!