  We present the results of a systematic search for multiperiodic pulsators
among the Galactic Bulge RR Lyrae stars of the OGLE-1 sample. We identify one
"canonical" double-mode variable (RRd star) pulsating in two radial modes. In
38 stars we detect secondary periodicities very close to the primary pulsation
frequency. This type of multiperiodic variables constitute ~23% of RRab and ~5%
of RRc population of the Bulge. With the observed period ratios of 0.95-1.02
the secondary periods must correspond to nonradial modes of oscillation. Their
beating with the primary (radial) pulsation leads to a long-term amplitude and
phase modulation, known as the Blazhko effect. The Blazhko RRab variables occur
more frequently in the Galactic Bulge than in the LMC. The opposite tendency is
seen in case of the RRd stars. The differences of incidence rates are most
likely caused by different metallicity of the two populations. We discuss
pulsation properties of the OGLE-1 Blazhko stars and compare them with
predictions of theoretical models. We argue, that the oblique magnetic pulsator
model of Shibahashi (2000) cannot account for the observations and should be
ruled out.
