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Rotational relaxation in the $v4=1$ out-of-plane bending mode of $A~1A2$ formaldehyde is investigated by means of a novel, folded double-resonance technique. A pulsed dye laser is tuned to coincide with a specific rovibronic transition in the $A~1A2\to A~1A1401$ band. The transient gain appearing on a copropagating cw probe laser, coincident with the $210310441$ transition, samples total depopulation rates as well as rotational state-to-state cross sections. Results verify that relaxation in the formaldehyde $A~1A2v4=1$ level occurs with a rate approximately ten times the gas kinetic collision rate. The sensitivity of transient gain spectroscopy, in conjunction with its superior spectral $(0.05cm-1)$ and temporal (6 nsec) resolution, is shown to be especially useful in the $H2COA~1A2$ system where the radiative rate is slow compared to the strongly rotation level dependent rates of nonradiative processes.