A dynamic probabilistic model has been designed for the personal computer to determine the technical and economic consequences of various biological variables and management strategies concerning reproduction and replacement in swine. In it, the Markov chain approach is used to simulate herd dynamics. The herd is described in terms of states animals can be in and the possible transitions between the states. The corresponding transition probabilities are derived from input values concerning biological variables and management strategies. The model has the property to calculate easily the herd structure in its stationary state. Sets of input values can be evaluated by comparing the results of the corresponding herds at equilibrium. Moreover, herd dynamics can be studied over a period of time to gain insight into how equilibrium is reached. Input values can be easily modified to be farm-specific. The number of states considered in the model is optional, making it possible to provide either aggregated or detailed simulations.