  We present preliminary results from the recent multi-epoch HI absorption
measurements toward several pulsars using the Arecibo telescope. We do not find
significant variations in optical depth profiles over periods of 4 months and 9
years. The upper limits on the optical depth variations in directions of
B0823+26 and B1133+16 are similar on scales of 10--20, 350 and 500 AU. The
large number of non detections of the tiny scale atomic structure suggests that
the AU-sized structure is not ubiquitous in the interstellar medium.
