In the present study we have investigated the acquisition of the past participle of Italian verbs of the second (including mostly irregular verbs) and third (including mostly regular verbs) conjugations in school age children, and with simulations with an artificial neural network. We aimed to verify the extent to which children are sensitive to regularity, as opposed to the consistency in the mapping from the infinitive to the past participle. In particular, we predicted that children would learn at some point that verbs of the second conjugation tend to be irregular, and therefore they would be more likely to produce irregularizations for verbs of this class, compared to the verbs of the third conjugation. However, they should also show sensitivity to the phonological mapping consistency within each subclass, learning to produce correct forms on the basis of phonological similarity. In contrast, children should be more likely to produce regular forms for verbs of the third conjugation. Thus, a larger regularity effect would be expected for verbs of the third than of the second conjugation, leading to the prediction of a regularity by conjugation interaction.