New object oriented simulation environments offer dramatically improved possibilities for simulation of coupled systems. In contrast to traditional building simulation, where separate, stand-alone tools are used for each simulation task; the new environments will offer a unifiedf ramework for all simulation problems. For design tool users this leads to two major advantages. (1) model coupling and comparison of results between different simulation applications will be practical, and (2) usage and input data will be standardised across applications. Thereby tool usability will increase considerably. Furthermore, man-time spent on model development and maintenance is expected to decrease, making it feasible to tailor appropriate models to each task and thus improve the quality of simulation results. In this paper, we report our experiences implementing a set of models for multi-zone air-exchange ME) in the general simulation environment IDA. Although MAE analysis is yet far from a standard industrial practice, several tailored tools have been developed. For comparison between general programs and specialised, the MAE models are extremely challenging, since the model structure can be utilised to an unusual degree in the specialised tools. In spite of this, the results obtained are encouraging in terms of processing time as well as numerical reliability (robustness). The paper presents a selection of the models, including the full Neutral Model Format (NMF) code, a discussion of numerical methods, and some general conclusions with bearing on simulation tool development.