The high-order harmonic generation process was studied
using laser pump pulses of different duration, down to
7 fs. The time profile of this kind of pulse corresponds
to a few optical cycles at the fundamental frequency, and
therefore a cycle-to-cycle variation of the maximum amplitude
occurs. These facts violate the adiabatic approximation
of the generation process, giving rise to new aspects of
the physical process. The comparison of results with pulses
in the two regimes are reported and discussed. The modeling
of the complete generation and propagation process in three
dimensions is also presented.