The brightness (cumulativelog N−log P) distribution provides information on the intrinsic and spatial properties of the gamma-ray burst (GRB) population. The brightness distribution for the whole sample of GRBs shows a clear deviation from a three-dimensional Euclidean distribution. However, when spectral/duration classes are considered, GRBs show quite distinct properties from theirlog N−log Pdistributions. We find that among the whole sample of GRBs of the 3B catalogue,only the long/hard GRBs show a strong deviation from the Euclidean distribution. This property is at odds with simple extragalactic models of GRB formation, unless a strong spectral and luminosity cosmological evolution is invoked. The existence of multiple GRB populations with different origins cannot be excluded. ©1998 American Institute of Physics.