Simulations with a Mars general circulation model (GCM) are used to perform idealized tracer transport experiments, which are analyzed to yield estimates of eddy mixing coefficients as well as ''stratospheric'' ventilation timescales for the zonal-mean circulation. The results indicate that relatively moderate values of the vertical eddy mixing coefficient, Kz~20--100 m2/s, may be most appropriate for the 10 to 45-km altitude region of the Martian atmosphere. Under dusty solstice conditions, somewhat stronger eddy mixing is present, but the transport is dominated by advection by the mean meridional circulation which acts to ventilate the atmosphere above 1 mbar (~20 km) in only about 7 days. Such a mean circulation has transport effects which are roughly comparable to those produced by eddy mixing with a much larger Kz value, ~1500 m2/s. In contrast, the computed mean ventilation timescale for a nondusty equinox circulation is approximately 180 days. In this case, vertical eddy mixing is the dominant transport process. In an intermediate nondusty solstice case, the effects of mean advection and eddy mixing are of comparable importance. An estimate of a mean ventilation timescale from the GCM for a late northern winter seasonal date (~45 days) is in very good agreement with the value of ~38 days recently inferred from Mariner 9 infrared imaging spectrometer (IRIS) data <Santee and Crisp, 1995>. ¿ American Geophysical Union 1996