In 377 we find him at Rome, where he appears to have gone on an embassy to Gratian, to whom he there delivered his oration entitled "Erotikos". On the association of Theodosius I in the empire by Gratian, at Sirmium, in 379, Themistius delivered an elegant oration, congratulating the new emperor on his elevation. Of his remaining orations some are public and some private; but few of them demand special notice as connected with the events of his life. In 384, (about the first of September), he was made prefect of Constantinople, an office which had been offered to him, but declined, several times before. He only held the prefecture a few months, as we learn from an oration delivered after he had laid down the office, in which he mentions, as he had done even six years earlier, and more than once in the interval, his old age and ill-health. From the thirty-fourth oration we also learn that he had previously held the offices of "princeps senatus" and "praefectus annonae", besides his embassy to Rome; in another oration he mentions ten embassies on which he had been sent before his prefecture; and in another, composed probably about 387, he says that he has been engaged for nearly forty years in public business and in embassies. So great was the confidence placed in him by Theodosius, that, though Themistius was not a Christian, the emperor, when departing for the West to oppose Magnus Maximus, entrusted his son Arcadius to the tutorship of the philosopher, 387-388. Nothing is known about Themistius after this time; and he may have died around 390. Besides the emperors, he numbered among his friends the chief orators and philosophers of the age, Christian and non-Christian. Not only Libanius, but Gregory of Nazianzus also was his friend and correspondent, and the latter, in an epistle still extant, calls him the "king of arguments."