The detector response package ( [HITS]) allows to establish a correspondence between the volumes seen by the particle and the active components of the detectors. When entering a new volume in GTRACK the subroutine GFINDS is called. If the volume has been declared by the user as a sensitive detector through appropriate calls to GSDET and if the corresponding tracking medium constant ISVOL is non zero, GFINDS fills the common block /GCSETS/ with the information to identify uniquely the detector component. This enables the user, in GUSTEP, to record the hits in the proper JHITS substructure [HITS].