The Latin phrase recited by Clerici on his way to kill the Quadris was "Animula, vagula, blandula, hospes comesque corporis", the first line of a poem attributed to the Roman emperor Hadrian.
When Clerici asks the operator to connect him with Prof. Quadri, the telephone number he gives is the (one-time) telephone number of Bertolucci's idol Jean-Luc Godard. When Quadri answers the phone, Clerici recalls one of his lectures in which Quadri said "The time for reflection is over. Now is the time for action." This is the opening line in Godard's film Le Petit Soldat.
Bertolucci proposed the film adaptation of the novel to Paramount, without ever reading it himself. Within a month, he started the screenplay.
The poem recited by Marcello while traveling on train is Gabriele D'Annunzio's "La pioggia nel pineto" (1903).