The revolt which now occupied for a time the attention of Mithridates was that of Hyrcania.  The Hyrcanians were Arians in race; they were brave and high-spirited, and under the Persian monarchs had enjoyed some exceptional privileges which placed them above the great mass of the conquered nations.  It was natural that they should dislike the yoke of a Turanian people; and it was wise of them to make their effort to obtain their freedom before Parthia grew into a power against which revolt would be utterly hopeless.  Hyrcania might now expect to be joined by the Medes, and even the Mardi, who were Arians like themselves, and could not yet have forgotten the pleasures of independence.  But though the effort does not seem to have been ill-timed, it was unsuccessful.  No aid was given to the rebels, so far as we hear, by any of their neighbors.  Mithridates’s prompt return nipped the insurrection in the bud; Hyrcania at once submitted, and became for centuries the obedient vassal of her powerful neighbor.

The conquest of Media had brought the Parthians into contact with the rich country of Susiana or Elymais; and it was not long before Mithridates, having crushed the Hyrcanian revolt, again advanced westward, and invaded this important province.  Elymais appears to have a had a king of its own, who must either have been a vassal of the Seleucidse, or have acquired an independent position by revolt after the death of Epiphanes.  In the war which followed between this monarch and Mithridates, the Elymseans proved wholly unsuccessful, and Mithridates rapidly overran the country and added it to his dominions.  After this he appears to have received the submission of the Persians on the one hand and the Babylonians on the other, and to have rested on his laurels for some years, having extended the Parthian sway from the Hindoo Koosh to the Euphrates.

The chronological data which have come down to us for this period are too scanty to allow of any exact statement of the number of years occupied by Mithridates in effecting these conquests.  All that can be said is that he appears to have commenced them about B.C. 163 and to have concluded them some time before B.C. 140, when he was in his turn attacked by the Syrians.  Probably they had been all effected by the year B.C. 150; since there is reason to believe that about that time Mithridates found his power sufficiently established in the west to allow of his once more turning his attention eastward, and renewing his aggressions upon the Bactrian kingdom, which had passed from the rule of Eucratidas under that of his son and successor, Heliocles.