There are three primary objectives of this experimental program: 1) to investigate in detail the interrelationships between growth (body weight, tail length, and tibial length) and pituitary and plasma growth hormone (GH) as measured by both the radioimmunoassay and biological or tibial assays, 2) to study the regulation of growth and GH by the external environment, and 3) to study the regulation of growth and GH by the internal environment. To accomplish these objectives both growth and growth hormone of rats subjected to alterations in photoperiod, olfaction, nutrition, and internal environment will be measured. The role of the pineal gland in mediating the response will be tested.