The wet and dry weights of parenchymatous (lung, heart, liver, kidney) and of mesenchymal organs (aorta, skin, xiphoid cartilage) of humans and rats (both sexes) were investigated during the course of life. Furthermore, these investigations were done on the following rat organs: brain (divided in cerebrum as well as in cerebellum plus pons), spleen, rib cartilage, testis and ovar. The water content of these tissues and organs of both species was calculated by means of their wet and dry weights. The dry weights of the investigated parenchymal and mesenchymal organs of both species remain constant during the whole course of life (pastly until the high senile age)--using freeze-drying. The same is true for the water content: constancy without significant decrease during the investigated life periods and without sex dependent differences.