(15th of 25) The fertile lowlands of Hyrania (Gurgan) were a highly permeable frontier with the nomadic tribes east of the Caspian. The Dahae had occupied the region in Achaemenian times from their plain of Dihistan. The Arpani clans then followed the road along the upper Atrek into Parthava, renaming that province Abarshahr, "realm of the Aparni." Perhaps some Sacaraucae migrated there in the 2nd century BC, or else it was the Dahae who founded Sace and Sinica. From the late 4th century AD, Chol and Kidarite Huns pressed upon the province from Dihistan and sought to follow the road to Mah. From the reign of Yazdgard II, periodic campaigns were necessary to contain their penetration within the province.

The city of Astarabad, Ptolemy's "metropolis of Hyrcania," lay on the Parthian main highway. It may also particularly in an independent Hyrcania after c. 50 AD, have shared in the trade which avoided Parthian control. The Kushans, as mediators of the Chinese and Indian overland trade, dealt with the Aorsi kingdom as well as with the Parthians. Goods were transported through Sughd to Khwarazm, then west to the Caspian coast; for the first stage, the Oxus may have been used, as well as the caravan routes. Hyrcania could have offered an alternative, land route to Armenia via Parishkhwar.