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 Q0; injecting into the conduit (15), at a flowrate Q1, 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 [B1sol]; injecting into the conduit (15), at a flowrate Q2, 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 [B2sol] different than the first concentration [B1sol] in the first concentrated solution; regulating the injection flowrate Q1 and the injection flowrate Q2 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.