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The ability of MSH and of methionine enkephalin (met-E) to induce dispersion of pigment granules was examined in melanophores and in xanthophores of the zebrafish Brachydanio rerio using the melanophore index (MI) and videodensitometry. Both methods gave similar results. In B. rerio both MSH and met-E induced pigment dispersion in dermal melanophores, in fin and peritoneal melanophores, and in xanthophores. Darkening lasted a few hours. However, met-E-induced darkening developed 40-50 min later and faded more slowly than the effect of MSH. Both effects were dose related. Naloxone prevented met-E-induced darkening while it did not interfere with the MSH-induced effect. Epidermal melanophores did not react to either MSH or met-E. Thus met-E proved to induce changes of coloration when injected into a fish. Our data suggest a central mechanism involved in met-E-mediated change of coloration in zebrafish under the conditions examined. A new approach was suggested for objective measurement of the mean body darkness of the fish with the help of computational videodensitometry. Our fist results indicate a proportionality between the MI evaluation and videodensitometry.

Secretion of oestrogen by the ovaries of foetal (15-19 days of gestation) and newborn rats in organ and tissue culture was not detectable by fluorometry when the ovary was taken from foetuses before folliculogenesis had occurred. In organ cultures of ovaries, the time of folliculogenesis corresponded with the normal timing of folliculogenesis in vivo. In tissue cultures the process of formation of follicles was delayed. Oestrogens were present in the medium when folliculogensis was fully established in the cultured foetal ovaries. Secretion began spontaneously and did nto depend on the addition of gonadotrophins to the medium. The addition of gonadotrophins the the culture medium did not effect the level of oestrogen secreted by the foetal and newborn rat ovaries during the period of incubation (2-3 weeks).

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