A model of dynamic oligopsony is estimated for the liquid milk market in the UK. The paper extends existing methods of estimating such models by allowing for the joint estimation of the market conduct equation and the input supply equation. This entails the estimation of a two equation model in which the parameters of one equation change between two regimes whilst those of the other do not. Our results provide little evidence of dynamic strategic behaviour and suggest that the farm-gate price of milk is determined competitively.