1. The pharmacokinetics of recombinant erythropoietin, r-Epo, were evaluated after intravenous and subcutaneous administration of 50 u kg-1 to six healthy male volunteers. 2. The calculated mean values (+/- s.d.) for volume of distribution at steady state and clearance after an i.v. dose were 76 (+/- 33) ml kg-1 and 12 (+/- 3) ml h-1 kg-1, respectively. 3. Serum concentrations of r-Epo peaked at 13 (+/- 6) h after the s.c. dose and the bioavailability over 72 h was 36 (+/- 23)%. The mean residence time and half-life of erythropoietin were 6.2 (+/- 1.0) and 4.5 (+/- 0.9) h respectively after i.v. and 46 (+/- 18) and 25 (+/- 12) h after s.c. administration. 4. The results demonstrate the possibility of changing the profile of the concentration-time curve by changing the mode of administration of r-Epo, with implications for the time-course of clinical response.