Media Atropatene (Azarbaijan) was never really under Seleucid rule. The rulers of Bacteria from the beginning were at least quasi-independent and in the middle of the 3rd century revolted and established absolute independence. At the same time Parthia under the leadership of the Arsacids cast off Seleucid rule and established a Parthian Empire as a sort of successor to the old Persian Empire. Although even under the greatest of the Parthians (Tiridates, Mithradates I, and Mithradates II) the realm did not have the old extent, it was formidable and was a rival to Rome.
The Romans in almost continuous warfare failed to halt the Persian drives to the west, which were often supported by local ambitious or frightened rulers under Rome. The Parthians' capital was the city of A Hundred Gates near Damqan, afterwards Rey near Tehran, and later on Hamadan, and in winter Ctesiphon near Baghdad. The Parthians ruled from 256 BC until 226 AD. The frontiers of Iran during the Parthians were: Transcaucasia in the north, India in the east, and the Tigris in the west. Parthians established their own customs duties, collecting 5% on imports.