Currently, the optimal dosing regimen for achieving and maintaining target Hb concentrations in various clinical settings remains incompletely defined. Both IV and SC routes of administration are used in the clinical setting and have been shown to be effective despite different bioavailability and pharmacokinetic profiles. This study is designed to describe the pharmacokinetic and pharmacodynamic profiles of several different epoetin alfa dosing regimens administered by both IV and SC routes in anemic critically ill patients admitted to a critical care area. The dosing regimens selected will be compared among themselves and against the 40,000 IU SC weekly dosing regimen (A) being used in a large registration trial. Specifically, the six dosing regimens were selected to gather PK and PD data about the following questions: 1) Will an early large Cmax, achieved by IV dosing, stimulate more reticulocytosis? (IV vs. SC dosing regimens A vs. B, C vs. D, E vs. F); 2) Do smaller more frequent doses of the same total dose result in the same PD profile? (A vs. C, B vs. D); 3) Does an IV load improve PD response? (E and F vs. C and D); 4) Do large frequent loading doses accumulate? (A vs. E and B vs. F). Results of this study will provide a pharmacokinetic foundation for understanding and potentially maximizing the pharmacodynamic effects of different dosing options in the critically ill patient. In order to maximize subject safety, all dosing will cease when subject's hemoglobin is > 13g/dL. Group A:40 K SC Qw: Days 1,8,15; Group B:40 K IV Qw: Days 1,8,15; Group C:15 K SC QOD: Days 1,3,5,7,9,11,13,15; Group D:15 K IV QOD: Days 1,3,5,7,9,11,13,15; Group E:40 K SC Days 1 and 3, then 15 K SC QOD: Days 5,7,9,11,13,15; Group F: 40 K IV Days 1 and 3, then 15 K SC QOD: Days 5,7,9,11,13,15