Cardiovascular and immune system abnormalities have been reported in females with estrogen deficiencies. To control these disorders in post-menopausal women, hormone replacement therapy (HRT) has been used. In the present study, we investigated the effects of HRT using estradiol or tibolone on the natriuretic peptide system and pro-inflammatory cytokines in ovariectomized rats. The procedures were carried out in compliance with the guidelines for the ethical use of animals in scientific research and were approved by the Ethical Committee of the Federal University of Espírito Santo. All of the surgical procedures were carried out under ketamine (70 mg/kg i.p.) and xylazine (10 mg/kg i.p.) anesthesia. Female rats were divided into four groups (n=7): SHAM, ovariectomized (OVX), OVX treated with 17beta-estradiol (OVX+E: 14 days 0.5 mg/kg/day) and OVX treated with tibolone (OVX+T: 14 days 1.5 mg/kg/day), 21 days after ovariectomy. On day 35, blood was collected to determine atrial natriuretic peptide (ANP) and cytokine (IL-6 and TNF-α) levels. In addition, selected tissues were collected for determining ANP, natriuretic peptide receptor type-A (NPR-A) and type-C (NPR-C) gene expression levels by RT-PCR. Values are means ± S.E.M., compared by ANOVA. The cytokine levels of IL-6 and TNF-α were increased in ovariectomized rats (OVX, IL-6: 32.80 ± 5.00 and TNF-alfa: 31.00 ± 5.00 vs SHAM, IL-6: 19.30 ± 6.00 and TNF-alfa: 20.30 ± 2.00 pg/ml). Here, we show that treatment with estradiol or tibolone decreased cytokine levels, except the level of IL-6 in the OVX+T group. In the ovariectomized, vehicle-treated rats, the plasma ANP concentrations were lower than in the SHAM group (OVX: 263.30 ± 53.30 pg/ml vs SHAM: 92.50 ± 19.50 pg/ml, P < 0.05). Treatment with estradiol or tibolone significantly increased the levels of ANP in the plasma of ovariectomized rats (OVX+E: 247.30 ± 29.90; OVX+T: 285.70 ± 60.40 pg/ml, P < 0.05 vs. OVX). Left atrium tissue (SHAM: 5.68 ± 0.35 vs OVX: 3.48 ± 0.40 microg/mg proteina P < 0.05) and plasma (OVX: 263.30 ± 53.30 pg/ml vs SHAM: 92.50 ± 19.50 pg/ml, P < 0.05) ANP concentrations were decreased after ovariectomy, as well as, ANP mRNA levels in the left atrium (OVX 40.00 ± 10.10 AU vs SHAM: 91.30 ± 4.40 AU) and the NPR-A mRNA levels in the kidney (OVX: 0.68 ± 0.11 vs SHAM group 12.00 ± 5.20 AU; P < 0.05). HRT normalized these parameters, except the levels of atrial ANP in the OVX+T group. The NPR-C mRNA levels in the kidney did not change. These data indicate that ovariectomy impairs the natriuretic peptide system and what appears to be one of the pathways that induces increased the levels of pro-inflammatory cytokines in this experimental model. Furthermore, our data suggest for the first time that treatment with tibolone has anti-inflammatory effects by influencing the natriuretic peptide system and the levels of TNF-alfa.