In this paper we describe fundamental properties of an “off‐line” three‐dimensional transport model, that is, a model which uses prescribed rather than predicted winds. The model is currently used primarily for studying problems of the middle atmosphere because we have not (yet) incorporated a formulation for the convective transport of trace species, a prerequisite for many tropospheric problems. The off‐line model is simpler and less expensive than a model which predicts the wind and mass evolution (an “on‐line” model), but it is more complex than the two‐dimensional (2‐D) zonally averaged transport models often used in the study of chemistry and transport in the middle atmosphere. It thus serves as a model of intermediate complexity and can fill a useful niche for the study of transport and chemistry. We compare simulations of four tracers, released in the lower stratosphere, in both the on‐ and off‐line models to document the difference resulting from differences in modeling the same problem with this intermediate model. These differences identify the price to be paid in going to a cheaper and simpler calculation. The off‐line model transports a tracer in three dimensions. For this reason, it requires fewer approximations than 2‐D transport model, which must parameterize the effects of mixing by transient and zonally asymmetric wind features. We compare simulations of the off‐line model with simulations of a 2‐D model for two problems. First, we compare 2‐D and three‐dimensional (3‐D) models by simulating the emission of an NOx‐like tracer by a fleet of high‐speed aircraft. The off‐line model is then used to simulate the transport of14C and to contrast its simulation properties to that of the host of 2‐D models which participated in an identical simulation in a recent NASA model intercomparison. The off‐line model is shown to be somewhat sensitive to the sampling strategy for off‐line winds. Simulations with daily averaged winds are in very good qualitative agreement but are less diffusive than when driven with instantaneous winds sampled at half‐hour intervals. Simulations with the off‐line and 2‐D models are quite similar in the middle and upper stratosphere but behave quite differently in the lower stratosphere, where the 3‐D model has a substantially more vigorous circulation. The off‐line model is quite realistic in its simulation of14C. While there are still systematic differences between the 3‐D calculation and the observations, the differences seem to be substantially reduced when compared with the body of 2‐D simulations documented in