They found a vent for their superabundant population in Seistan, Afghanistan, and India, and ceased to have any hopes of making an impression on the Arsacid kingdom.
Mithridates, it is probable, even took territory from them.

The acquisition of parts of Bactria by the Parthians from the Scyths, which is attested by Strabo, belongs, in all likelihood, to his reign; and the extension of the Parthian dominion to Seistan may well date from the same period.

Justin tells us that he added many nations to the Parthian Empire.

The statements made of the extent of Parthia on the side of Syria in the time of Mithridates the First render it impossible for us to discover these nations in the west: we are, therefore, compelled to regard them as consisting of races on the eastern frontier, who could at this period only be outlying tribes of the recent Scythic immigration.
The victories of Mithridates in the East encouraged him to turn his arms in the opposite direction, and to make an attack on the important country of Armenia, which bordered his north-western frontier.

Armenia was at the time under the government of a certain Ortoadistus, who seems to have been the predecessor, and was perhaps the father, of the great Tigranes.