An ultrashort chirped pulse propagating in an optically dense atomic medium may undergo strong modulation on time scale shorter than its duration. We report in this paper a theoretical and experimental study of these reshaping effects. We demonstrate that two kinds of interferences are involved. The first one is due to the interference between the electric field radiated at resonance and that radiated off resonance, inducing thus a modulation in the time profile of both the radiated field and the excited state population. The second one results from the interference between the radiated field and the incident one. Moreover, we show that after a transient time of the order of 2rootdelta(-1) where delta is the chirp parameter, the reshaping effects observed on the output intensity result from only the interference between the resonant part of the radiated field and the incident pulse. (C) 2003 Elsevier B.V. All rights reserved.