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The effects of oestrogens on the reproductive ducts and copulatory structures of female higher vertebrates are well known: these results are supplemented by the present work in which pellets of oestradiol have been implanted intramuscularly in female dogfish Scyliorhinus caniculus at different stages of sexual maturity. This treatment was found to have a pronounced stimulatory effect on the oviducts of certain of the treated fish which was obvious on macroscopic examination. Secretion of horny egg-case material was especially stimulated. Sexually immature specimens were not affected and the oviducts of mature egg-laying fish did not appear to show increased stimulation. The group which responded to the implanted steroid appeared to be about to mature, though vitellogenesis had not yet started in the most stimulated specimen. The cloacal region appeared also to be stimulated in the fish of this particular group. Rate of absorption of the oestradiol from the implant was low, the most stimulated fish having absorbed approximately 3 mg. in seven months. It is suggested that at this low rate of absorption the stimulatory effects are likely to be of a physiological nature. These results acquire additional significance in the light of the recent discovery of oestradiol 17β in the ovaries of Squalus suckleyi.