  Black hole mass determination in active galaxies is a key issue in
understanding various luminosity states. In the present paper we try to
generalise the mass determination method based on the X-ray excess variance,
successfully used for typical broad line Seyfert 1 galaxies (BLS1) to Narrow
Line Seyfert 1 (NLS1) galaxies. NLS1 galaxies differ from BLS1 with respect to
several properties. They are generally more variable in 2-10 keV energy band so
the natural expectation is the need to use a different scaling coefficient
between the mass and the variance in these two types of sources. However, we
find that such a simple approach is not enough. Although for majority of the 21
NLS1 galaxies in our sample a single scaling coefficient (larger by a factor
20) provided us with a satisfactory method of mass determination, in a small
subset of NLS1 galaxies this approach failed. Variability of those objects
appeared to be at the intermediate level between NLS1 and BLS1 galaxies. These
exceptional NLS1 galaxies have much harder soft X-ray spectra than majority of
NLS1 galaxies. We thus postulate that the division of Seyfert 1 galaxies into
BLS1 and NLS1 according to the widths of the Hbeta line is less generic than
according to the soft X-ray slope.
