  We investigate the constraints imposed on the luminosity function (LF) of
long duration Gamma Ray Bursts (LGRBs) by the flux distribution of bursts
detected by the GBM at ~1 MeV, and the implications of the non detection of the
vast majority, ~95%, of the LGRBs at higher energy, ~1 GeV, by the LAT
detector. We find a LF that is consistent with those determined by BATSE and
Swift. The non detections by LAT set upper limits on the ratio R of the prompt
fluence at ~1 GeV to that at ~1 MeV. The upper limits are more stringent for
brighter bursts, with R<{0.1,0.3,1} for {5,30,60}% of the bursts. This implies
that for most bursts the prompt ~1 GeV emission may be comparable to the ~1 MeV
emission, but can not dominate it. The value of R is not universal, with a
spread of (at least) an order of magnitude around R~10^(-1). For several bright
bursts with reliable determination of the photon spectral index at ~1 MeV, the
LAT non detection implies an upper limit to the ~100 MeV flux which is <0.1 of
the flux obtained by extrapolating the ~1 MeV flux to high energy. For the
widely accepted models, in which the ~1 MeV power-law photon spectrum reflects
the power-law energy distribution of fast cooling electrons, this suggests that
either the electron energy distribution does not follow a power-law over a wide
energy range, or that the high energy photons are absorbed. Requiring an order
unity pair production optical depth at ~100 MeV sets an upper limit for the
Lorentz factor, Gamma<=10^(2.5).
