The goal of the proposed research is to contribute to our understanding of the hormonal control of anuran tadpole metamorphosis and the role of light in modulating metamorphic rate. The pineal gland, which secretes the hormone melatonin on a rhythmic basis, is believed to integrate photoperiodic input with neuroendocrine functions in amphibians as well as other vertebrates. Since light, the length of the photoperiod, and the overall daylength affect the rate of tadpole development, melatonin may be involved in the transfer of environmental information to the hormonal axis (thyroid, primarily) controlling metamorphosis. One approach in this research is to study the effect of melatonin Rana pipiens growth and development, on metamorphic hormone responses of intact tadpoles, and on the history and rhythmicity of cell proliferation in the thyroid gland. A specific aim of this part of the proposed work will be to see if melatonin mediates the negative effect of darkness or short photoperiods on tadpole metamorphic rate. Adrenal cortical steroids increase in the plasma of tadpoles during metamorphosis. They appear to synergize with thyroid hormones to promote metamorphic events in older tadpoles, while they inhibit the process at young stages. The pattern of rise of concentration of the various steroids during metamorphosis is different, and thus the hormones may have different functions. Another aim of this project is to administer various corticoids to young tadpoles to see if the inhibition of metamorphosis is correlated with a change in thyroid gland histology or cell proliferation, and to older tadpoles with and without thyroid hormone to study the possible effects of the corticoids on hindlimb development and skin differentiation. This research will contribute to knowledge of metamorphosis, a developmental phenomenon controlled by hormones and influenced by environmental factors, including the light/dark cycle. It should also increase our understanding of how light exerts its effect on organisms, the interrelationships of hormone action, and limb development and skin differentiation.