A skin pricker has a barrel (1,11) in which a hammer (2,29) released by a trigger mechanism (32,34) can be shot forwards by a compressed spring (3,38) to impact on a lancet (24) and momentarily project its tip. The rear end of the spring acts against a barrier (4,39) adjustable axially of the barrel, so that the spring can be more or less compressed before release, causing the hammer to act with greater or lesser force on the lancet. The barrier (4,39) may have projections (6,8,42) that engage in skew slots (7,9,43) in the barrel, so that rotation of the barrier axially adjusts it. A sleeve (44) over the rear part of the barrel instrumental in priming the device may co-operate with the projections and be rotatable to set the desired spring force.