The results of a search for ultrarapid variability of galactic water maser sources at the wavelength 1.35 cm on a timescale of several minutes to 1 h are reported. A special observational technique was developed. Using this technique, several observational sessions for more than 30 maser sources were carried out from 2002 to 2006. Among those, several sources that have shown such variability were found. The causes of this are discussed. The most interesting of these causes are internal processes within the sources ongoing in regions with a characteristic size of about 0.1 AU. Clear evidence for such a variability type was found in at least two sources: W49N and W33B. Variability of this kind displays itself only in certain states of the water masers.