The petrogeny of the leucite-bearing rocks occurring in the Roman and Birunga volcanic regions is reappraised using previously published data. It is shown by means of Principal Latent Vector Variation Diagrams (Le Maitre, 1968) that the chemical analyses of rocks from the two provinces form complementary suites within a well defined trend, which can be interpreted in terms of low-pressure crystal differentiation. The potential role of the basic compositions of the trend as parent material is examined in the light of recent experimental evidence and a possible evolutionary model is proposed involving polybaric crystallization.