The Bloch equations are solved for a two-level system interacting with
a strong pulsed laser pump and a weak pulsed laser probe. It is shown
that the four-wave-mixing excitation spectrum, which is symmetrical w
ith respect to the pump-probe detuning delta in the steady-state regim
e, becomes asymmetrical under pulsed conditions. The asymmetry is ascr
ibed mainly to the linewidths of the pump and probe laser pulses.