A process for the make-up of a catalyst in a reactor suitable for reactions which take place in three-phase systems according to the Fischer-Tropsch technique, to compensate losses (in activity and material) during the overall production cycle, which comprises: a) incorporating the catalyst, previously reduced in a matrix of paraffinic waxes, solid at room temperature; b) melting and collecting the paraffinic matrix in a vessel, maintained at a high temperature, together with a diluent which is miscible with the molten paraffinic matrix and which is in liquid form both under the conditions present in the vessel and at room temperature, a stream of inert gas being distributed in said vessel from the bottom so as to obtain a sufficiently homogeneous suspension; c) pressurizing the vessel in which the complete melting of the paraffinic matrix has been effected at a pressure higher than that of the conditioning reactor maintaining the system fluidized by the continuous introduction of inert gas from the bottom of said vessel; d) transferring, due to the pressure change, the diluted solution from the vessel under pressure to the reactor, initially empty, maintained at a temperature higher than or equal to that present in the vessel and flushed in turn from the bottom with inert gas; e) regulating the temperature and pressure in the reactor at values ranging from 200-230°C and 0.5-1.5 MPa; f) gradually substituting the inert gas with synthesis gas up to a concentration of inert gas ranging from 5 to 50% by volume and maintaining a partial water pressure (co-product of the Fischer-Tropsch synthesis reaction) lower than 1.0 MPa; g) maintaining the conditions of point (f) for 24-72 hours; h) gradually increasing the pressure inside the reactor to a value higher than the pressure of the reactor; i) gradually reducing the concentration of inert gas to zero; j) gradually increasing the reaction temperature until reaching values ranging from 200 to 350°C; k) after completing the conditioning phase, transferring the suspension from the reaction vessel to the main reactor, which is running under normal operating conditions, by means of a pressure change. a) incorporating the catalyst, previously reduced in a matrix of paraffinic waxes, solid at room temperature; b) melting and collecting the paraffinic matrix in a vessel, maintained at a high temperature, together with a diluent which is miscible with the molten paraffinic matrix and which is in liquid form both under the conditions present in the vessel and at room temperature, a stream of inert gas being distributed in said vessel from the bottom so as to obtain a sufficiently homogeneous suspension; c) pressurizing the vessel in which the complete melting of the paraffinic matrix has been effected at a pressure higher than that of the conditioning reactor maintaining the system fluidized by the continuous introduction of inert gas from the bottom of said vessel; d) transferring, due to the pressure change, the diluted solution from the vessel under pressure to the reactor, initially empty, maintained at a temperature higher than or equal to that present in the vessel and flushed in turn from the bottom with inert gas; e) regulating the temperature and pressure in the reactor at values ranging from 200-230°C and 0.5-1.5 MPa; f) gradually substituting the inert gas with synthesis gas up to a concentration of inert gas ranging from 5 to 50% by volume and maintaining a partial water pressure (co-product of the Fischer-Tropsch synthesis reaction) lower than 1.0 MPa; g) maintaining the conditions of point (f) for 24-72 hours; h) gradually increasing the pressure inside the reactor to a value higher than the pressure of the reactor; i) gradually reducing the concentration of inert gas to zero; j) gradually increasing the reaction temperature until reaching values ranging from 200 to 350°C; k) after completing the conditioning phase, transferring the suspension from the reaction vessel to the main reactor, which is running under normal operating conditions, by means of a pressure change.