We consider the general problem of the meridional distribution and seasonal variability of long-lived atmospheric constituents. Here ''long-lived'' means that the time scale of the constituent exceeds the atmospheric exchange time across the region of interest. The relevant time scale is the ''dynamic lifetime,'' defined here as the ratio of the total atmospheric loading of the constituent to the total global source. Provided that this quantity is large (greater than about 5 years for the troposphere), the constituent attains a ''gradient equilibrium'' in which spatial gradients of mixing ratio are weak and the meridional and seasonal structure take on a form which, for a given source, is independent of the dynamic lifetime. Analysis of this structure involves clarification of the nature of atmospheric exchange processes and leads us to highlight the nonuniqueness of such concepts as ''interhemispheric exchange times.'' These results are illustrated using a two-dimensional transport model to conduct simulations of idealized long-lived constituents with, in most cases, a simple mid-latitude northern hemisphere source and a sink with uniform decay rate. It is suggested that these results will aid the synthesis of observations of long-lived tropospheric constituents and will simplify the validation of transport models. More generally, this kind of approach might help to identify the effects of transport on trace constituent structures and thereby contribute to the understanding of the distribution of constituents which are not ''long-lived'' in the sense used here. ¿ American Geophysical Union 1988