  The rare and conspicuous flux density variations of some radio sources
(extragalactic and pulsars) for periods of weeks to months have been denoted
Extreme Scattering Events (ESE's) by Fiedler et al. (1987). Presently, there is
no astrophysical mechanism that satisfactorily produces this phenomenon. In
this paper, we conjecture that inhomogeneities of the electronic density in the
turbulent interstellar medium might be the origin of this phenomenon. We have
tested this conjecture by a simulation of the scintillation of the pulsar
B1937+21 at 1.4 GHz and 1.7 GHz for a period of six months. To this end, we
have constructed a large square Kolmogorov phase screen made of 131k x 131k
pixels with electron inhomogeneity scales ranging from 6 x $10^6$ m to
$10^{12}$ m and used the Kirchhoff-Fresnel integral to simulate dynamic spectra
of a pulsar within the framework of Physical Optics.
  The simulated light curves exhibit a 10 day long variation simultaneously at
1.41 and 1.7 GHz that is alike the ``ESE'' observed with the Nancay
radiotelescope toward the pulsar B1937+21 in October 1989. Consequently, we
conclude that ``ESE'' toward pulsars can be caused naturally by the turbulence
in the ionized interstellar medium instead of invoking the crossing of discrete
over pressured ionized clouds on the line of sight as in the model of Fiedler
et al. (1987). We suggest that longer events could occur in a simulation of
scintillation, if larger electron inhomogeneities > $10^{12}$ m were included
in the construction of the Kolmogorov phase screen. This next step requires a
supercomputer.
