One-piece fitting for fastening a horizontally extending, split running rail formed from at least two running rail sections to a horizontal top panel and/or to a vertical middle wall of a sliding door cabinet, wherein a rail leg, which is horizontal in the mounted position, of the running rail and/or a rail leg, which is vertical in the mounted position, of the running rail comprise(s) at least one running track for a sliding door, comprising - two parallel spaced apart side walls, which form between them an insertion opening being open to the rear, to the top and to the bottom in the mounted position and which are vertical in the mounted position, for inserting the fitting onto the middle wall, - at each of the two sides of the insertion opening an abutment surface, which extends at right angles to the side walls and is horizontal in the mounted position, for abutment against the lower side of the top panel, - at each of the two sides of the insertion opening at least one fastening opening for the frontal screwing on of a running rail section, - two receiving sections, which are arranged below and in front of the horizontal abutment surface and are upwardly open in the mounted position, for inserting the leg ends of the rail legs, which are horizontal in the mounted position, of the running rail sections, and - a middle section arranged between the two receiving sections and having a middle leg projecting vertically in front of the insertion opening in the mounted position, wherein the rail legs, which are horizontal in the mounted position, of the running rail sections abut against the middle section and the vertical rail legs abut against the middle leg, and wherein the middle section and/or the middle leg comprise(s) at least one running track which connects the at least one running rack of the received running rail sections to form a continuous running track. - two parallel spaced apart side walls, which form between them an insertion opening being open to the rear, to the top and to the bottom in the mounted position and which are vertical in the mounted position, for inserting the fitting onto the middle wall, - at each of the two sides of the insertion opening an abutment surface, which extends at right angles to the side walls and is horizontal in the mounted position, for abutment against the lower side of the top panel, - at each of the two sides of the insertion opening at least one fastening opening for the frontal screwing on of a running rail section, - two receiving sections, which are arranged below and in front of the horizontal abutment surface and are upwardly open in the mounted position, for inserting the leg ends of the rail legs, which are horizontal in the mounted position, of the running rail sections, and - a middle section arranged between the two receiving sections and having a middle leg projecting vertically in front of the insertion opening in the mounted position, wherein the rail legs, which are horizontal in the mounted position, of the running rail sections abut against the middle section and the vertical rail legs abut against the middle leg, and wherein the middle section and/or the middle leg comprise(s) at least one running track which connects the at least one running rack of the received running rail sections to form a continuous running track.