With a radioimmunoassay for melatonin, the secretional characteristics of serum melatonin in the ricefield eel, Monopterus albus Zuiew were studied. The results show that serum melatonin levels in the ricefield eels demonstrated a clear daily cycle and changed seasona1ly. The diel rhythm was characterized by a rapid increase almost immediately (0. 5- 1 hour)after the onset of darkness,maintaince of high levels for the duration of the night,and a rapid decline to basal levels (about 1/2- 1/3 of the nighttime amplitude)at the time of light onset. Dramatic seasonal variation in amplitude of the nocturnal increase was found in the serum too. The serum melatonin levels of female ricefield eels increased significantly accompanied with ovarian development,and reached its highest level at the fourth stage of ovarian maturation, but then decreased rapidly at the prespawning stage. After spawning, serum melatonin levels elevated again. During sex-reversal process, serum melatonin levels decrease gradually, and the lowest values were recorded in the mature males. The seasonal rhythm suggests that endogenous melatonin appears to be involved in the process of sex reversal in the ricefield eel.