Transport and mixing in the extratropical stratosphere of the NASA Goddard Space Flight Center (GSFC) three-dimensional (3-D) chemical transport model (CTM) are evaluated using the modified Lagrangian-mean diagnostic. The normalized equivalent length squared (ξ) has been calculated from simulated N2O and CH4 distributions using two different wind data sets and has been compared with that calculated from observations by the CLAES satellite instrument. There is generally good agreement, indicating that the CTM realistically simulates the location and seasonal evolution of mixing regions/barriers in the extratropical lower and middle stratosphere. Differences that occur between the CTM fields and observations at high latitudes in winter and spring can be attributed to interannual variability in polar meteorology. There are also some differences in northern summer: the CTM series shows regions of high ξ in the extratropics that are not observed in ξ from CLAES. These high values occur where there is small-scale variability in N2O in regions with very weak meridional gradients. Comparison of ξ from isentropic tracer simulations using the same wind fields as used in the 3-D CTM show good agreement except during the summer. The isentropic tracers do not have as much small-scale variability as the 3-D CTM tracer fields, indicating that the enhanced small-scale structures in the CTM N2O, and high values of ξ, are produced by small-scale horizontal variability in the vertical motions. The fact that vertical motions can influence summer values of ξ means that these ξ fields cannot be interpreted in terms of quasi-horizontal mixing.