This paper examines the behaviour of two firms competing in a duopoly, where firms can influence demand through use of advertising. The paper simulates the strategic interaction of the two firms based on a game-theoretic Cournot analytical model. The evolution over time of the Nash equilibrium is graphically displayed for a number of different competitive scenarios. The results show that there exist threshold levels of advertising effectiveness at which duopoly behaviour bifurcates, that perfectly cooperative advertising can lead to competitive disadvantage, and that perfectly predatory advertising can lead to stagnation or losses.