The monarchy was elective, but only in the house of the Arsacidae; and the concurrent vote of both councils was necessary in the appointment of a new king.

Practically, the ordinary law of hereditary descent appears to have been followed, unless in the case where a king left no son of sufficient age to exercise the royal office.
Under such circumstances, the Megistanes usually nominated the late king's next brother to succeed him, or, if he had left behind him no brother, went back to an uncle.

When the line of succession had once been changed, the right of the elder branch was lost, and did not revive unless the branch preferred died out or possessed no member qualified to rule.

When a king had been duly nominated by the two councils, the right of placing the diadem upon his head belonged to the Surena, the "Field-Marshal," or "Commander in Chief of the Parthian armies." The Megistanes further claimed and sometimes exercised the right of deposing a monarch whose conduct displeased them; but an attempt to exercise this privilege was sure to be followed by a civil war, no monarch accepting his deposition without a struggle; and force, not right, practically determining whether he should remain king or no.
After a king was once elected and firmly fixed upon the throne, his power appears to have been nearly despotic.

At any rate he could put to death without trial whomsoever he chose; and adult members of the Royal House, who provoked the reigning monarch's jealousy, were constantly so treated.