A smooth start-up is crucial to the successful operation of a SAGD process. A simulation study was conducted to analyze the start-up or circulation period of SAGD well pairs. A coupled reservoir/wellbore model was developed and used to history match field data obtained from wells with extensive instrumentation at Petro-Canada's MacKay River development. The history matched model was then used to conduct a sensitivity study oil some of the parameters that affect the circulation of a well pair. First, the steam-to-toe time was examined for the two typical wellbore completions that Petro-Canada employs at MacKay River. Various flow rates were tested for the two completions to see how the steam injection rate affected the steam-to-toe time. Next, the circulation pressure along with the injector/producer pressure gradient was investigated. Various pressure gradients were applied between the injector and producer to examine the effects on the circulation duration and the formation of steam coning. Finally, the distance between the injector and producer was studied. The purpose was to explore the effect of the vertical separation between the injector and producer wells on conversion time.