An extensible leash for pets comprises a shell (1, 101) provided with a handle (1, 102), a cable-winding roll (3, 103) provided with a cable (6, 22) with an end integral with the roll (3, 103) and a free end external to the shell (1, 101). The roll (3, 103) is rotatable around a rotation axis as a result of the action of a spiral-shaped spring (5, 105) Means for locking/unlocking the roll are provided with a control mechanism operated by a first lever (2, 106) to be operated by the palm of the hand to move from an unlocking position to a locking position of the roll (3, 103). The first lever (2, 106) is associated with a bi-stable locking device (11, 107) adapted to hold the first lever locked in the two positions and to allow it to move from the locking position to the unlocking position.