Experimental design and plasma Epo levels in response to rhEpo treatment. (A) Summary of the experimental procedures. The participants were subcutaneously injected with rhEpo 5000 IU on each occasion: every other day for 14 days and weekly during the last 2 weeks (large arrows). RBCV indicates total red blood cell volume. All of the measurements were made just before the next rhEpo injection. (B) The time course of plasma Epo in response to rhEpo treatment. Additional blood sampling on day 21 indicates the level of plasma Epo before the rhEpo injection on day 21. However, the absence of a measurement on day 23 prevented us from addressing the response of plasma Epo 48 hours after the previous rhEpo injection on day 21. Values are mean plus or minus SD for 8 participants. The statistical differences from pre-rhEpo values were calculated with the Wilcoxon test; *P < .05.