The Syro-Macedonian monarchs could always count with certainty on having powerful friends in Parthia, whatever portion of it they invaded; and even the Romans, though their ethnic connection with the cities was not so close, were sometimes indebted to them for very important assistance.
We are told that Mithridates I., after effecting his conquests, made a collection of the best laws which he found to prevail among the various subject peoples, and imposed them upon the Parthian nation.

This statement is, no doubt, an exaggeration; but we may attribute, with some reason, to Mithridates the introduction at this time of various practices and usages, whereby the Parthian Court was assimilated to those of the earlier Great Monarchies of Asia, and became in the eyes of foreigners the successor and representative of the old Assyrian and Persian Kingdoms.

The assumption of new titles and of a new state--the organization of the Court on a new plan--the bestowal of a new character on the subordinate officers of the Empire, were suitable to the new phase of its life on which the monarchy had now entered, and may with the highest probability, if not with absolute certainty, be assigned to this period.
It has been already noticed that Mithridates appears to have been the first Parthian sovereign who took the title of "King of Kings." The title had been a favorite one with the old Assyrian and Persian monarchs, but was not adopted either by the Seleucidae or by the Greek kings of Bactria.

Its revival implied a distinct pretension to that mastery of Western Asia which had belonged of old to the Assyrians and Persians, and which was, in later times, formally claimed by Artaxerxes, the son of Sassan, the founder of the New Persian Kingdom.

Previous Parthian monarchs had been content to call themselves "the King," or "the Great King"-- Mithridates is "the King of Kings, the great and illustrious Arsaces." At the same time Mithridates appears to have assumed the tiara, or tall stiff crown, which, with certain modifications in its shape, had been the mark of sovereignty, both under the Assyrians and under the Persians.