This paper attempts to cast light to the effect of monopoly regulation in Cournot markets compared to its effect in Bertrand markets. To this purpose, we use a simple model of a vertically linked market, where an upstream regulated natural monopoly is trading via two‐part tariff contracts with a downstream duopoly. Combining our results to those of the existing literature on deregulated markets, we argue that when the downstream competition is in prices, efficiency dictates regulating the monopoly with a marginal cost based pricing scheme. However, this type of regulation leads to significant welfare loss, when the downstream market is characterized by Cournot competition.