It has been shown previously in the rat that plasma erythropoietin (EPF) rises significantly in a short period of time (4-8 hours) in response to a hypoxic stimulus. This rise in EPF titer reaches a maximum at 24 hours and drops back to normal at 48 hours. The return to normal occurs in spite of the fact that the animal remains in a hypoxic atmosphere. Others have found this same general time response pattern. New Zealand white rabbits were placed in a low O2 chamber to find out whether a different species of animal would respond to a hypoxic stimulus in a different way and to establish the time after the onset of hypoxia when maximum values of EPF could be found. At the termination of each time interval, rabbits were removed from the chamber and immediately exsanguinated by bleeding via heart puncture. The plasma was prepared for assay by the heat deproteinization procedure of Borsook, et al. and by utilizing the step 1 procedure of White, et al. The final material was concentrated three times as compared to the original plasma volume. The plasma extract thus prepared was assayed in starved rats using the method of Fried, et al, and Mirand et al. There was significant elevation of plasma EPF levels at 8 hours. Peak levels were present at 24 and 48 hours. By 72 hours the level of erythro-poietin was back to normal levels and remained there up to 120 hours. These results demonstrate that the rabbit and the rat respond similarly to a hypoxic stimulus. The only demonstrable difference is that erythro-poietin levels remain elevated slightly longer in the rabbit than the rat.