A theoretical, numerical, and experimental study is made of the behavior of the ponderomotive effect for rf frequencies close to the particle gyrofrequency. Near gyroresonance the stopping potential is found to be small rather than infinite. The interaction is not adiabatic, and heating results both in the parallel and perpendicular directions. This behavior is explainable in terms of the finite transit time of the particle through the localized rf structure. A perturbation theory that includes finite transit time is found to be in good quantitative agreement with both the numerical study and the experimental results.