The activity of the aspartate-, alanine-, tyrosine-, phenylalanine- and tryptophane aminotransferases in the rat organes in development have been investigated by quantitative histochemical methods. The isoenzymes have also been examined. The variable increase of the aminotransferase activity has been observed in the liver, brain, heart, skeletal muscle and kidney. In spite of the differences of the aspartate aminotransferase activity in the organs, the increase up to the 7th postnatal day, the reduction after that and the repeated increase after the 14th day reaching the level of the adult animals is evident as a common trend. A considerable increase of the alanine aminotransferase activity has been observed in the late postnatal period. While the difference in the activity of the aromatic aminotransferases in the embryonic organs is small, the changes of the 3 enzymes are different in the postnatal development. The number and the intensity of the isoenzymes of the aspartate- and alanine aminotransferases increase in the development. The isoenzyme spectrum of aromatic aminotransferases in the embryo proves an equal in number and intensity of fractions. In the development this similarity is preserved only with regard to cathode isoenzymes, while with anode once some differences appear.