Concentrations of CH3CN and HCN have been measured in the lower stratosphere by aircraft-based ion-molecule reaction mass spectrometry. The mean HCN volume mixing ratio of 164 parts per trillion by volume (pptv) is consistent with previous infrared remote sensing data (160 pptv). The measured CH3CN volume mixing ratios markedly exceed most previous lower stratospheric values and reveal a significant decrease from 160 to 110 pptv between 1.0 and 4.2 km above the tropopause. This latter feature suggests a stratospheric CH3CN lifetime much shorter than thought previously, which can be explained by an additional stratospheric sink, maybe ion-catalyzed conversion of CH3CN to HCN. Our present data indicate a mean global tropospheric CH3CN source of 1.6¿1012 g CH3CN per year which is slightly larger than the CH3CN source inferred from most previous atmospheric measurements of CH3CN but is in agreement with estimations inferred from laboratory simulation experiments of biomass burning. ¿ 1997 American Geophysical Union