The rulers of the Sasanian Empire (224–654 CE) were natives of that south-western region, and through their political and cultural influence, Middle Persian became a prestige dialect and thus also came to be used by non-Persian Iranians.
The earliest texts in Zoroastrian Middle Persian were probably written down in late Sassanid times (6th–7th centuries), although they represent the codification of earlier oral tradition.
Even less-well attested are the Middle Persian compositions of Nestorian Christians like Mar Maʿna, evidenced in the Pahlavi Psalter (7th century); these were used until the beginning of the second millennium in many places in Central Asia, including Turfan (in present-day China) and even localities in Southern India.
The corpus of medieval texts of Zoroastrian tradition include around 75 works, of which only a few are well known: A manuscript known as the "miscellaneous codex" or MK (after Mihraban Kaykhusrow, the Indian Zoroastrian (Parsi) copyist who created it), dated to 1322 but containing older material, is the only surviving source of several secular Middle Persian works from the Sassanian period.
Among the texts included in the unique MK are: Especially important to cultural and law historians is the Madayan i Hazar Dadestan, "Book of a Thousand Judgements", a 7th-century compilation of actual and hypothetical case histories collected from Sassanian court records and transcripts.