Circadian variations in circulating plasma sex steroids in female catfish, Clarias batrachus (Linn.).

Estradiol (E2) and 17 alpha-hydroxyprogesterone (17 alpha-OHP) levels exhibited significant circadian rhythms in the prespawning phase of the reproductive cycle. Pattern of rhythm was unimodal. Whereas, in the spawning phase E2 levels did not show significant fluctuations in their daily cycle whereas 17 alpha-OHP levels showed significant rhythm (P less than 0.001). Amplitude of rhythm of E2 was greater in prespawning phase while 17 alpha-OHP showed greater amplitude in spawning phase.