Analysis of UBV data obtained in 1984–2001 for NGC 4151 (2nd activity cycle after a long 5-year minimum) shows the very different nature of slow and fast variations. This conclusion has been drawn from the analysis of color indices U-B and B-V of variable source in the nucleus of NGC 4151. The ascending branch of the light curve from the minimum in 1984–1989 to maximum in 1995 shows the increasing of temperature from 6000–7000 K up to 40,000–50,000 K with the brightness of the variable source increasing from 3–5 to 35–40 mJy. After the maximum (1995̈C1997), the strong UV excess appeared, while the range of B-V changes was the same, i.e., the temperature changes were the same as in the ascending branch. The slow component can be connected with transport of matter into accretion disk and its heating. The flare component has two main properties: 1) the majority of points are located on two-color diagram near the locus of the hot stars or black body with temperature of ~ 50, 000 K, and 2) the duration of brightness increase does not depend on the flare amplitude and is equal to 23-25 days. On the other hand, it is known that the dimensions of active region effectively emitting in the optical are of the order of 1–3 light days. So, the main cause of flares must be a shock wave with the velocity of 10000–15000 km/s.

These results together with some other facts strongly support the model of disk accretion onto the supermassive black hole.