Bayesian networks are parametric models for multidimensional domains exhibiting complex dependencies between the dimensions (domain variables). A central problem in learning such models is how to regularize the number of parameters; in other words, how to determine which dependencies are significant and which are not. The normalized maximum likelihood (NML) distribution or code offers an information-theoretic solution to this problem. Unfortunately, computing it for arbitrary Bayesian network models appears to be computationally infeasible, but we show how it can be computed efficiently for certain restricted type of Bayesian networks.