Summary Blood volume measurements using Cr51 and I131 were carried out before and after severe, gradually corrected blood loss in unanesthetized sheep. Reversibly and irreversibly shocked animals were studied. Blood volume and total body hematocrit remained unchanged; large vessel hematocrit rose, and Fcells ratio fell. Excessive plasma skimming in the small vessels is suggested as the possible cause of this phenomenon. The unpredictability of Fcells ratio in posthemorrhagic states and the fallacy inherent in the use of a single label in blood volume determinations is reemphasized.