A model coupling photochemistry and vertical transport in the troposphere has been used to investigate the magnitude of possible future perturbations to atmospheric CH4. We have studied the response of atmospheric CH4 to an increase in the concentration of CO or to a variation in stratospheric O3. Both of the above mechanisms, which could be caused by man's activities, arise because a change in the concentration of tropospheric OH will lead to a change in the CH4 abundance. Our calculations imply that a perturbation of ~30-40% in atmospheric CH4 could occur if stratospheric O3 were perturbed by 10% or if man-made CO continued to increase. The possible consequences of a CH4 perturbation may entail a perturbation in stratospheric photochemistry or in the thermal balance of the atmosphere.