The brook lamprey, Lampetra planeri, has a larval period of approximately 5½ yr., after which it metamorphoses during the autumn months into the adult form. It reaches sexual maturity the following May, spawns and dies (Hardisty, 1961). This metamorphosis is remarkable in that it always occurs at the same time of the year and that, from any one locality, all the ammocoetes which are ready to undergo metamorphosis begin to do so together. This suggests a precise controlling mechanism though its identity has not, as yet, been established. Attempts have been made to accelerate metamorphosis by administration of thyroid (Remy, 1922; Horton, 1934) and pituitary (Young & Bellerby, 1935; Knowles, 1941) extracts but these have all failed. Sterba (1955) has obtained a partial stimulation of metamorphosis by injecting larval stages of L. planeri with corticotrophin. During a previous investigation (Eddy & Strahan, 1968) both normal and pinealectomized ammocoetes of Geotria