An intravenous pharmacokinetics for a new red cell substitute, PEG-hemoglobin SB1, was studied in SD rats. Total-hemoglobin and its metabolite methemoglobin in the plasma were determined using a spectrophotometer. The limit of quantitation was 0.01g/dL and the C.V for interday assay reproducibility was less than 6%. Upon intravenous administration of anticipated clinical dose, 10 ml(0.7 gHb)/kg, plasma concentration curve of total-hemoglobin was well described by one-compartment model. The $t_{frac{1}{2}},{;}CL_{t}$, Vd and $AUC^{0-48hr}$ were $8.23{pm}0.96{;}hr,{;}0.06{;}{pm}{;}0.01 {;}dL/hr/kg,{;}0.66{pm}0.05{;}dL/lg{;}and{;}13.6{;}{pm}{;}1.01g{cdot}hr/dL$, respectively, in male rats(n=5, $mean{;}{pm}{;}SD$). Those parameters in female rats were $9.21{;}pm{;}2.31{;}hr,{;}0.06{;}{pm}{;}0.01{;}dL/hr/kg,{;}0.79{pm}{;}0.08{;}dL/kg{;}and{;}13.0{;}{pm}{;}2.36{;}g{cdot}hr/dL$, respectively. Similar kinetic profiles between males and females were also obtained from other parameters. Small amount of methemoglobin, an oxidative metabolite of SB1, was detected in the plasma of both sexes, where the $AUC^{0-48{;}hr,m}$ and $t_{{frac{1}{2}},m}$ were approximately $1.5{;}g{cdot}hr/dL$ and 20 hr, respectively. The present work provides a critical kinetic data for the effective clinical applications of PEG-hemoglobin SB1.