The device for conditioning preforms in plastic material comprises a rotating turret (20), having a group of conditioning cavities (22) for inserting preforms, partly through an opening on a first side (A) of the turret and partly through an opening on a second side (B) of the turret. The conditioning cavities (22) which open on the first side (A) are disposed in such a way that for part of their length (L1) they are situated side by side with the cavities which open on the second side (B). In this way the turret becomes more compact. There is also described a conditioning method for the preforms.