GLOBAL surface-based measurements of atmospheric methane and carbon monoxide concentrations revealed a marked and unexpected decrease in their growth-rates in 1991 and 1992, particularly in the Northern Hemisphere(1,2). Changes in emissions are unlikely to be the sole reason for the sudden reduction in the concentrations of these source gases(2,3). The unprecedentedly large depletion of Stratospheric ozone observed in 1991 and 1992 (ref. 4) may have contributed to the sharp decrease in the growth rates of both CH4 and CO by exposing the troposphere to more ultraviolet radiation. This would have resulted in increased concentrations of the hydroxyl radical, OH., which is the major atmospheric sink for both CH4 and CO. Here we present two-dimensional model simulations which allow us to assess the significance of the link between stratospheric ozone depletion and the observed trends of CH4 and CO. We find that the low values in stratospheric ozone concentration can account for almost half of the 1992 decrease in the CH4 and CO growth rates.