An extensible leash for pets, comprising: - a shell (1, 101) provided with a handle (1, 102); - a cable-'winding roll (3, 103) housed in said shell (1, 101) and provided with a cable (6, 22) with an end integral with said roll (3, 103) and a free end external to said shell (1, 101) and sliding through an opening of said shell (1, 101) to be hooked up to a collar, a vest or any other item wearable by the animal, said roll (3, 103) being rotatable around a rotation axis as a result of the action of a spiral-shaped spring (5, 105) into said shell (1, 101), said spiral spring (5, 105) is housed in concentric position inside said cable-winding roll (3, 103); - means for locking/unlocking said roll provided with a control mechanism operated by a first lever (2, 106) arranged on said handle (1, 102) in such a position as to be operated by closing the palm of the hand to move from an unlocking position wherein said roll (3, 103) is free to rotate to a locking position of said roll (3, 103); wherein_said first lever (2, 106) is associated with a bi-stable locking device (11, 107) adapted to hold said first lever locked in the two positions and to allow it to move from the locking position to the unlocking position and vice versa only after a further pressure of said first lever (2, 106), said first lever (2, 106) has a first end pivoted on said handle to rotate around a pin (4, 113) and a second end (9, 116) adapted to interacting with said bi-stable device (11, 107), said first end of said first lever (2, 106) being connected to a lever mechanism, provided with a hook adapted to engage said cable-winding roll (3, 103) as a result of a rotation promoted by said lever mechanism due to the passage of said first lever (2, 106) from the unlocking position to the locking position to lock the rotation of said roll (3, 103); characterized in that said bi-stable device comprises a snap mechanism provided with a slider slidable in a fixed guide cylinder and adapted to engage, after a predetermined minimum stroke, a stop pin provided with cam means adapted to turn the rectilinear sliding movement of said slider into a rotation movement, said stop pin being provided with a pre-charged spring to promote the return movement of said slider toward said unlocking position, wherein said second end is provided with a nose integral with the bi-stable device (11, 107) so that said second end is locked in a position determined by the slider 108. - a shell (1, 101) provided with a handle (1, 102); - a cable-'winding roll (3, 103) housed in said shell (1, 101) and provided with a cable (6, 22) with an end integral with said roll (3, 103) and a free end external to said shell (1, 101) and sliding through an opening of said shell (1, 101) to be hooked up to a collar, a vest or any other item wearable by the animal, said roll (3, 103) being rotatable around a rotation axis as a result of the action of a spiral-shaped spring (5, 105) into said shell (1, 101), said spiral spring (5, 105) is housed in concentric position inside said cable-winding roll (3, 103); - means for locking/unlocking said roll provided with a control mechanism operated by a first lever (2, 106) arranged on said handle (1, 102) in such a position as to be operated by closing the palm of the hand to move from an unlocking position wherein said roll (3, 103) is free to rotate to a locking position of said roll (3, 103); wherein_said first lever (2, 106) is associated with a bi-stable locking device (11, 107) adapted to hold said first lever locked in the two positions and to allow it to move from the locking position to the unlocking position and vice versa only after a further pressure of said first lever (2, 106), said first lever (2, 106) has a first end pivoted on said handle to rotate around a pin (4, 113) and a second end (9, 116) adapted to interacting with said bi-stable device (11, 107), said first end of said first lever (2, 106) being connected to a lever mechanism, provided with a hook adapted to engage said cable-winding roll (3, 103) as a result of a rotation promoted by said lever mechanism due to the passage of said first lever (2, 106) from the unlocking position to the locking position to lock the rotation of said roll (3, 103); characterized in that said bi-stable device comprises a snap mechanism provided with a slider slidable in a fixed guide cylinder and adapted to engage, after a predetermined minimum stroke, a stop pin provided with cam means adapted to turn the rectilinear sliding movement of said slider into a rotation movement, said stop pin being provided with a pre-charged spring to promote the return movement of said slider toward said unlocking position, wherein said second end is provided with a nose integral with the bi-stable device (11, 107) so that said second end is locked in a position determined by the slider 108.