A method and a device for the admixing of treating agent to a pulp suspension. The pulp and agent are supplied through inlets (3 and 5) to a housing (1) with a chamber (10) comprising a substantially cylindric rotor (2). The mixing is carried out in a mixing zone (11) where the pulp is subjected to kneading between mixing members (6) on the rotor (2) and stationery treating members (12) in the chamber (10). After mixing, the pulp is discharged through an outlet (4).