The invention relates to a method for dissolving cationic starch. In the method is obtained an aqueous polyelectrolyte solution comprising a synthetic cationic polymer, which has a charge density value of at least 0.1 meq/g, determined at pH3, the concentration of the cationic polymer in the polyelectrolyte solution being > 2.5 weight-%, and the aqueous polyelectrolyte solution is brought together with cationic starch having a degree of substitution, DS, > 0.1. The cationic starch is dissolved to the polyelectrolyte solution by heating and/or mixing. The invention relates also to a papermaking agent comprising 2 – 40 weight-% of synthetic cationic polymer which has a charge density value of at least 0.1 meq/g, determined at p H 3, and 2.5 – 25 weight-% cationic starch having degree of substitution, DS, > 0.1. The papermaking agent is in slurry form and has a viscosity of 50 – 20 000 m Pas, measured at 25°C with Brookfield DVI+ viscometer.