Some of the most frequently presented scenarios for the initial power excursion of the Chernobyl accident are evaluated based on computer simulations. The applied transient model uses one-dimensional descriptions of the reactor core and the main flow circuit. According to the simulations, a slow flow decrease caused by gradual slowing down of the four main circulation pumps could have initiated the accident only if the void reactivity coefficient had been considerably larger than the original Soviet figure. On the other hand, a faster flow reduction, such as pump cavitation or deliberate stopping of even some of the pumps, would have produced enough void for prompt criticality. However, this scenario is sensitive to the size of the void coefficient and to the amount of flow reduction. The most probable initiator was considered to be the positive scram caused by the graphite followers of the manual control rods. Such a mechanism would naturally have brought the additional reactivity to the bottom half of the reactor, and the timing of the power surge would have been the reported one. The simulations indicated that the positive scram was possible only because of the double-humped axial power profile that probably prevailed in the reactor before the accident. The simulations also demonstrated the inability of the shutdown system in this sequence.