A radioimmunological method was developed for determining total and free corticosterone in rat plasma. This method was used to determine the dose-response curve of corticosterone and to measure the elimination and study the half-lives of total and free corticosterone in rat plasma with a dose of 5 mg/kg. The elimination with a dose of 5 mg/kg, when drawn on the half-logarithmic scale, formed a straight line. The half-lives for total and free corticosterone were 25 and 15 min, respectively.