  To study some characteristics of the interstellar medium, observational data
of pulsars with large Faraday rotation values (|RM|> 300 rad / m2) were used.
It was suggested and justified that large |RM| values can be due to the
contribution of the regions with increased electron concentration, projected on
the pulsar. Most likely these are the HII regions, dark nebulae and molecular
clouds. In these objects the magnetic field can be oriented in the direction of
a large-scale field of the Galaxy, or simply is a deformed extension of the
galactic field. It was shown that the Galactic distribution of rotation
measures of pulsars with |RM|> 300rad / m2 corresponds to the circular model of
the magnetic field of the Galaxy, with the counter-clockwise direction of the
magnetic field in the galactocentric circle 5 kpc <R <7 kpc.
