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Using two-color pump-probe spectroscopy in a magnetic field, we have measured the time-resolved cyclotron resonance of photogenerated transient carriers in undoped and doped InSb quantum wells. We used an intense femtosecond pulse of near-infrared (NIR) radiation from a Ti:sapphire-based regenerative amplifier to create a large density of nonequilibrium carriers, which modifies the transmission of a delayed pulse of far-infrared (FIR) radiation from a free-electron laser. We monitored the dynamics of FIR transmission while varying the magnetic field and the time delay between the NIR and FIR pulses. Our data clearly show that the average electron cyclotron mass decreases as the electrons relax towards the band edge, as expected from the strong nonparabolicity of the InSb conduction band. Detailed lineshape analysis combined with Landau level calculations allowed us to determine the time evolution of the Fermi-Dirac distribution function of nonequilibrium. two-dimensional carriers in a quantizing magnetic field.