A method in a reeler and a reeler, wherein a reeling nip (N1) is formed by a reeling core (5) or a growing machine reel (R) and at least one loop of an endless supporting member (1), which is substantially continuous in the axial direction of the reeling core (5). In the method the tension profile of the endless supporting member (1) is determined from the measured variables proportional to the tension of the endless supporting member (1).