We develop a model of Parametric Probabilistic Transition Systems. In this model probabilities associated with transitions may be parameters, and we show how to find instances of parameters that satisfy a given property and instances that either maximize or minimize the probability of reaching a given state. We show, as an application, the model of a probabilistic non repudiation protocol. The theory we develop, allows us to find instances that maximize the probability that the protocol ends in a fair state (no participant has an advantage over the others).