Protein patterns during final differentiation of some rat organs.

1. Protein composition of rat brain, and of heart and skeletal muscle was analysed by polyacrylamide gel electrophoresis in their late fetal and postnatal development. 2. Adult organs show specificity of their protein patterns as judged from the relative quantitative proportions and distribution of separated protein bands. 3. Protein composition of morphologically already well defined organs in the late fetal stage of development is not changed by the event of birth and is preserved unmodified until the early neonatal period. 4. Significant developmental changes in protein composition leading to formation of adult pattern take place between the 3rd and 30th postnatal day. 5. Results and limitations of the technique applied are evaluated from the point of applicability to developmental studies.