Testosterone and 17 beta-estradiol plasma fluctuations throughout spawning period in male and female rainbow trout, Oncorhynchus mykiss (Walbaum), kept under several photoperiod regimes <B>Testosterone and 17 beta-estradiol plasma fluctuations throughout spawning period in male and female rainbow trout, Oncorhynchus mykiss (Walbaum), kept under several photoperiod regimes</B><P> M. Pavlidis(1), D. Dimitriou & A. Dessypris</B><I><P> Pavlidis , M., Dimitriou D. & Dessypris, A., Division of Biochemistry, Cell and</I><I><BR> Molecular Biology and Genetics, Department of Biology, Athens University,</I><I><BR> Panepistimiopolis, Athens 15701, Greece. - (1)Present address: Institute of Marine</I><I><BR> Biology of'Crete, P, 0.</I> <I>Box 2214, Iraklion 71003, Crete, Greece</I><P> Received 2 October 1993, Accepted 28 January 1994<P> Plasma levels of testosterone (T) and 17 beta-estradiol (E2) were measured in rainbow trout during the course of their first and second spawning period. Trout were exposed to normal photoperiod (Group C) and two static, artificial light regimes (Group A, 2OL:4D from January to 15 May, then 4L:20D; Group B, 4L:20D from March to August, then 2OL:4D). Identical patterns of E2 fluctuations were observed in all groups; E2 levels rose from 54 days prior to spawning and kept rising to reach a peak at 18 days prior to spawning, then decreased steadily reaching basal levels at ovulation. T levels rose from 54 days prior to spawning and peaked 6-8 days prior to spawning and then declined sharply before ovulation, reaching basal levels 6-12 days post spawning. T in male trout peaked 12-0 days prior to milt production. No correlation was found between the steroids levels and percentage of ovulated and spermiated fish. No difference was observed in plasma E2 levels carrying ripe or overripe eggs. T and E2 fluctuations were also observed at the population level (both mature and non-mature fish). These findings support the use of plasma T and E2 levels as an ancilliary predictor index/tool for the initiation of the spawning period and for<B> </B>the determination of the timing for human intervention (stripping).<I><P> Ann. Zool. Fennici 31:319-327</I> <P>