The relative effect of parenteral versus oral admn. of cobaltous chloride on erythrocyte and Hb formation in rats was detd. Daily subcut. injn. of 2.5 mg./kg. of body wt., over a period of 6 wks., resulted in an increase of more than 30% in the number of erythrocytes/c. mm. and in gs. of Hb/100 ml. of blood. To produce a similar effect from oral admn. of cobaltous chloride, within the same period of time, 40 mg./kg. of body wt. was required. These dosages are without significant toxic effects. Rats which received 10 mg. cobaltous chloride/ kg. daily, subcut., averaged 24% wt. loss by the end of 6 wks. Rats which received 40 mg./kg. body wt., subcut., did not survive beyond 8 days.