We simulate collisions of brown dwarfs and massive planets with the cores of AGB stars, inside common envelopes. Each simulation is performed by injecting the companion's mass into the radiative zone between the core's edge and the base of the envelope convective zone. The simulations are performed with a spherically symmetric code. The important result of such a collision for the AGB star is the decrease in radius and luminosity. The faster the collision (higher mass injection rate) and the closer to the core it takes place, the more prominent are the effects implied by the collision. A brown dwarf of 0.01 M0, destroyed within 10 yr at a distance of 0.2 R0 from the AGB core, causes the AGB star's radius and luminosity to decrease by a factor of -2. The AGB stars recover their imtial structure less than 20 yr after mass injection ceases. The effects of such a collision on the AGB structure imply a significant decrease in the mass-loss rate, and can lead to the formation of a shell of thickness -2>c 10- pc in the descendant planetary nebula. Such a thin shell is very difficult to detect. We expect, however, the effects on the mass4oss geometry to be more prominent. Key words: stars: AGB and post-AGB - binaries: close - stars: evolution - stars: lowmass, brown dwarfs.