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Results of a long-term spectral monitoring of the active galactic nucleus ofNGC 4151 are presented (11 years, from 1996 to 2006). High quality spectra(S/N>50 in the continuum near Halpha and Hbeta) were obtained in the spectralrange ~4000 to 7500 \AA, with a resolution between 5 and 15 A, using the 6-mand the 1-m SAO's telescopes (Russia), the GHAO's 2.1-m telescope (Cananea,Mexico), and the OAN-SPM's 2.1-m telescope (San-Pedro, Mexico). The observedfluxes of the Halpha, Hbeta, Hgamma and HeII emission lines and of thecontinuum at the observed wavelength 5117 A, were corrected for the positionangle, the seeing and the aperture effects.  We found that the continuum and line fluxes varied strongly (up to a factor6) during the monitoring period. The emission was maximum in 1996-1998, andthere were two minima, in 2001 and in 2005. The Halpha, Hgamma and He II fluxeswere well correlated with the Hbeta flux. We considered three characteristicperiods during which the Hbeta and Halpha profiles were similar: 1996-1999,2000-2001 and 2002-2006. The line to continuum flux ratios were different; inparticular during the first period, the lines were not correlated with thecontinuum and saturated at high fluxes. In the second and third period, wherethe continuum flux was small, the Halpha and Hbeta fluxes were well correlatedto the continuum flux, meaning that the ionizing continuum was a goodextrapolation of the optical continuum. The CCFs are often asymmetrical and thetime lags between the lines and the continuum are badly defined indicating thepresence of a complex BLR, with dimensions from 1 to 50 light-days.

Long-term variability of the optical spectra of NGC 4151: I. Light curves and flux correlations