1. Studies were undertaken to examine the effect of acute and chronic administration of human recombinant erythropoietin on kidney cortical and papillary perfusion in the anaesthetized rat using laser-Doppler flowmetry. 2. Thirty minutes after erythropoietin (50 and 150 units/kg intravenously), blood pressure, cortical perfusion and papillary perfusion were unchanged. 3. In animals treated chronically with erythropoietin over 7 days (three doses of 150 units/kg subcutaneously) blood pressure was similar to that of vehicle-treated animals, whereas cortical perfusion and papillary perfusion were reduced by 23% and 20%, respectively (both P < 0.05), and the packed cell volume (51.1 +/- 0.7%) was significantly (P < 0.01) greater than in vehicle-treated animals (46.2 +/- 0.6%). 4. Bolus doses of vasopressin and phenylephrine increased blood pressure (by between 10% and 40%) and decreased cortical and papillary perfusion (by between 10% and 20%), while angiotensin II caused similar increases in blood pressure and decreases in cortical perfusion but not papillary perfusion. The magnitude and pattern of these responses were comparable after both acute and chronic administration of erythropoietin. 5. Erythropoietin given acutely at therapeutic levels has a marginal effect on cortical and papillary perfusion. However, the chronic treatment indicated that there was a sustained reduction in both cortical and papillary perfusion, reflecting a vasoconstriction. This reduction in renal haemodynamics could contribute to the increase in blood pressure observed when this hormone is administered in man.