In 1520 he went to Rome, where he entered the brilliant literary circle of Leo X.
When Charles of Bourbon stormed Rome in 1527 Paleario went first to Perugia and then to Siena, where he settled as a teacher. In 1536 his didactic poem in Latin hexameters, De immortalitate animarum, was published at Lyons. It is divided into three books, the first containing his proofs of the divine existence, and the remaining two the theological and philosophical arguments for immortality based on that postulate. The whole concludes with a rhetorical description of the occurrences of the Second Advent. In 1542 a tract, written by him and entitled Della Pienezza, sufficienza, et satisfazione della passione di Christo, or Libellus de morte Christi, was made by the Inquisition the basis of a charge of heresy, from which, however, he successfully defended himself. In Siena he wrote his Actio in pontijices rpmuiws et eorum asseclas, a vigorous indictment, in twenty "testimonial, " against what he now believed to be the fundamental error of the Roman Church in subordinating Scripture to tradition, as well as against various particular doctrines, such as that of purgatory; it was not, however, printed until after his death.
In 1546 he accepted a professorial chair at Lucca, which he exchanged in 1555 for that of Greek and Latin literature at Milan. Here about 1566 his enemies renewed their activity, and in 1567 he was formally accused by Fra Angelo the inquisitor of Milan. He was tried at Rome, condemned to death in October 1569, and executed in July 1570.