  We study changes in the $\gamma$-ray intensity at very high energies observed
from selected active galactic nuclei. Publicly available data collected by
Cherenkov telescopes were examined by means of a simple method utilizing solely
the number of source and background events. Our results point to some degree of
time variability in signal observed from the investigated sources. Several
measurements were found to be excessive or deficient in the number of source
events when compared to the source intensity deduced from other observations.
