The impact of uncontrolled charging of plug-in electric vehicles (PEVs) on distribution networks is investigated using a probabilistic approach based on Monte Carlo simulations. A model simulating daily residential and commercial electrical demand estimates the existing demand on the networks. A PEV operator model simulates the actions of drivers throughout a typical day to estimate the demand for vehicle charging. Three networks are studied that are typical of suburban, urban and rural networks, respectively. The analysis is focused on peak demand increases, secondary transformer overloading and voltage drops in the networks. PEV charging significantly increases the peak demand on all networks causing larger voltage drops and increasing the probability of transformer overloading.