In order to provide an accurate basis for the correlation of endocrine changes and phases of the reproductive (and associated) cycles in the powan of Loch Lomond, 'Coregonus- lavaretus (L.), the following experiments were carried out, Cell types in the pituitary were identified using light microscopy and transmission electron microscopy. To locate gonadotropic cells an immunofluorescence technique was employed using antibody prepared in rabbit and an immunodiffusion method to detect the presence of antibody in the rabbit serum. In order to purify pituitary gonadotropin, two purification techniques 'were used. One was alcoholic extraction followed by gel filtration on sephadex; the other was buffer extraction followed by affinity chromatography on Concanavalin A-sepharose. To characterise the fractions from columns, two in vitro bioassays were employed; (i) ovulation of the oocytes of medaka (Oryzias latipes); (ii) Cyclic AMP production in immature trout gonads. The concentration of pituitary gonadotropins was measured using a heterologous radioimmunoassay technique. Using a radioimmunoassay technique, the serum testosterone and oestradiol-17β concentrations were determined. Serum proteins were characterised using SDS gel electrophoresis.