  It is known that the dependence of the emission-line luminosity of a typical
cloud in the active galactic nuclei broad-line regions (BLRs) upon the incident
flux of ionizing continuum can be nonlinear. We study how this nonlinearity can
be taken into account in estimating the size of the BLR by means of the
"reverberation" methods. We show that the BLR size estimates obtained by
cross-correlation of emission-line and continuum light curves can be much (up
to an order of magnitude) less than the values obtained by reverberation
modelling. This is demonstrated by means of numerical cross-correlation and
reverberation experiments with model continuum flares and emission-line
transfer functions and by means of practical reverberation modelling of the
observed optical spectral variability of NGC 4151. The time behaviour of NGC
4151 in the H_alpha and H_beta lines is modelled on the basis of the
observational data by Kaspi et al. (1996, ApJ, 470, 336) and the theoretical
BLR model by Shevchenko (1984, Sov. Astron. Lett., 10, 377; 1985, Sov. Astron.
Lett., 11, 35). The values of the BLR parameters are estimated that allow to
judge on the size and physical characteristics of the BLR. The small size of
the BLR, as determined by the cross-correlation method from the data of Kaspi
et al. (1996, ApJ, 470, 336), is shown to be an artifact of this method. So,
the hypothesis that the BLR size varies in time is not necessitated by the
observational data.
