Erythropoietin (EPO) has several biological activities that induce not only erythrocytosis, but a pressor effect and the modulation of certain vasoactive substances. In this study, we investigated the role of heme oxygenase (HO) in erythrocytosis and the pressor mechanism during chronic EPO treatment. Male Sprague-Dawley rats were subcutaneously injected with EPO (300 IU/kg BW) three times a week for 6 weeks. The mRNA and protein expression levels of two isoforms of HO (HO-I and HO-II) in the kidney, liver and spleen were then measured and the renal function was estimated. The EPO-treated rats had higher hematocrit levels (68 ± 2% in EPO-treated, 43 ± 1% in control) and higher systolic blood pressures (160 ± 14 mmHg in EPO-treated, 120 ± 3 mmHg in control) (n=6, p<0.01). The expression of HO-I mRNA was significantly increased in the kidney (1.6 fold), the liver (2.4 fold) and the spleen (1.3 fold) in the treated rats, while HO-II was expressed at a constant level. The administration of antisense oligodeoxynucleotide (ODN) (100 μg/kg BW) increased the mean arterial pressure to a greater degree in the EPO-treated rats than in the control animals (n=6, p<0.01). The expression of HO-I protein was significantly down-regulated in the kidney and spleen but not in the liver. These observations suggest that HO-I may contribute to a depressor effect in the hypertensive state that is induced by EPO treatment.