In rabbits pregnancy causes a marked increase in the renin concentration of plasma and several organs: uterus, vagina, mesometrium, liver, ureter, heart, arteries and veins, muscle and fat tissue, while the renin content of the kidney, oviduct, ovary and adrenal gland is the same in pregnant and non-pregnant rabbits. There is evidence for a local renin formation in kidney, uterus, oviduct and in the chorion and amnion. The occurrence of renin in the other tissues may either be a result of local formation or due to uptake of circulating renin. The rabbit brain is not found to contain measurable amounts of renin.