Procedure for adding a filler into a pulp based on cellulose fibres, characterized in that the pulp is at a medium consistency and the pulp is fluidized, whereupon calcium hydroxide is [...] added into it and t h e pulp i s s ti rr e d in i t s fluidized state while the calcium carbonate is precipitated by introdu ci n g carbon d i ox ide into t h e pulp .