Plane constant‐pressure shock waves were used to initiate wedge‐shaped pressings of PETN (pentaerythritol tetranitrate). The shocks entered the PETN from brass or Lucite plates. Shock pressures in the plates and depths at which the PETN was initiated were measured with a streak camera. It was found that a 50‐kbar shock in the brass was barely sufficient to initiate granular PETN pressed to a density of 1.0 g/cm3. This corresponded to a derived pressure of about 2½ kbar in the PETN pressing. It was further shown that the interstitial gas had no effect on the depth of initiation.