In a total of 4 different studies, 2 involving 2 generations, the essentiality of Ni could be shown by reduced growth in response to a diet with 15 ppb Ni. In 30 day old self-reared rats, anemia was induced in the Ni-deficient animals despite a high Fe supply of 50 mg Fe/kg diet. In the F1 generation of the Ni-deficient animals, the erythrocyte count had fallen by 36%, the hematocrit by 37%, and the Hb content by 44%. In the F2 generation in which the animals were given 100 ppm Fe, surpassing their requirement 3 times, the blood parameters of the deficient animals were reduced by 8-10%. At the age of 50 days of the F1 generation the erythrocyte count had fallen 23% compared to the values of the control group, the hematocrit 14%, and the Hb content (16%) from 12.7-10.7 g/100 ml blood. An influence on the protein content of the serum and on the catalase activity of the erythrocytes was not observed and only at times was there an influence on the urea content of serum.