Abstract: The present invention relates to a device for controlling the working fluid circulating in a closed circuit (10) operating according to a Rankine cycle, said circuit comprising a heat exchanger (22) for evaporation of said fluid, swept by a hot fluid (Ch) from a hot source (28), expansion means (30) for expanding the fluid in vapour form, a cooling exchanger (40) swept by a cold fluid (Fr) for condensation of the fluid in vapour form, and a circulation and compression pump (12) for the fluid in liquid form. According to the invention, the device comprises means (54) of managing the mass of fluid contained in circuit (10).