  We study the faint end of the HI mass function (HIMF) in order to test the
predictions of the CDM theory on the number density of objects with small
(dark) masses. The neutral hydrogen is much better tracer of the underlying
mass distribution compared to the luminous matter and can be used to test the
existence of a population of small galaxies in which the star formation has
been partially or completely suppressed during cosmic evolution. Due to
technical limitations, the existing HI surveys are not very sensitive to HI
masses below 10^8 M_sun. We designed a blind HI survey to be sensitive to
objects with small HI masses. The surveyed area is in the Canis Venatici groups
of galaxies and covers in total ~ 86 deg^2 of sky, with observed velocities in
the range -350 < cz < 1400 km/s. We detected 69 objects, 22 of them for the
first time in HI. All new HI detections fall in the lower part of the
mass-histogram, confirming our ability to detect galaxies with small HI masses.
The calculated HIMF is flat in the faint end regime (slope ~ -1), different
from the steep rise predicted by CDM models. Possible effects of the
environment on the estimated HIMF parameters are discussed.
