The Parthian institutions are referred to Mithridates I., rather than to Tiridates, because in the reign of Mithridates Parthia entered upon a new phase of her existence--became an empire instead of a mere monarchy; and the sovereign of the time could not but have reviewed the circumstances of his State, and have determined either to adopt the previous institutions of his country, or to reject them.

Mithridates I.had attained a position which entitled and enabled him to settle the Parthian constitution as he thought best; and, if he maintained an earlier arrangement, which is uncertain, he must have done so of his own free will, simply because he preferred the existing Parthian institutions to any other.

Thus the institutions may be regarded as starting from him, since he approved them, and made them those of the Parthian EMPIRE.
Like most sovereignties which have arisen out of an association of chiefs banding themselves together for warlike purposes under a single head, the Parthian monarchy was limited.

The king was permanently advised by two councils, consisting of persons not of his own nomination, whom rights, conferred by birth or office, entitled to their seats.

One of these was a family conclave (concilium domesticum), or assembly of the full-grown males of the Royal House; the other was a Senate comprising both the spiritual and the temporal chiefs of the nation, the Sophi, or "Wise Men," and the Magi, or "Priests." Together these two bodies constituted the Megistanes, the "Nobles" or "Great Men"-- the privileged class which to a considerable extent checked and controlled the monarch.