A method for preparing a medical liquid from a liquid, such as water, and two concentrated solutions comprises the following steps: circulating the liquid in a conduit ( 15 ), at a flowrate Q 0； injecting into the conduit ( 15 ), at a flowrate Q 1 , a first concentrated solution containing a first ionic substance A and a second ionic substance B, the ionic substances A and B having, respectively, in the first concentrated solution, a concentration [Asol] and a first concentration [B 1 sol]； injecting into the conduit ( 15 ), at a flowrate Q 2 , a second concentrated solution containing the first ionic substance A and the second ionic substance B, the first ionic substance A having, in the second concentrated solution, the same concentration [Asol] as in the first concentrated solution, and the second ionic substance B having, in the second concentrated solution, a second concentration [B 2 sol] different than the first concentration [B 1 sol] in the first concentrated solution； regulating the injection flowrate Q 1 and the injection flowrate Q 2 of the first and second concentrated solutions in such a way that at any given time the diluted solution resulting from the mixing of the liquid and the concentrated solutions has a desired concentration [Ades] of first substance A and a desired concentration [Bdes] of second substance B.
