ATREK, a river which rises in lat. 37° 1 o' N., long. 59° E., in the mountains of the r_orth-east of the Persian province of Khora san, and flows west long the borders of Persia and the Turk menistan S.S.R. (q.v.). After a course of 35om., it enters the south-eastern corner of the Caspian sea, its mouth being in the Turkmenistan S.S.R., a little north-north-west of the Persian town of Astarabad.