Background Marinobufagenin, NKA (Na/K‐ATPase) inhibitor, causes vasoconstriction and induces fibrosis via inhibition of Fli1 (Friend leukemia integration‐1), a negative regulator of collagen synthesis. In vascular smooth muscle cells (VSMC), ANP (atrial natriuretic peptide), via a cGMP/PKG1 (protein kinase G1)‐dependent mechanism, reduces NKA sensitivity to marinobufagenin. We hypothesized that VSMC from old rats, due to downregulation of ANP/cGMP/PKG‐dependent signaling, would exhibit heightened sensitivity to the profibrotic effect of marinobufagenin. Methods and Results Cultured VSMC from the young (3‐month‐old) and old (24‐month‐old) male Sprague–Dawley rats and young VSMC with silenced PKG1 gene were treated with 1 nmol/L ANP, or with 1 nmol/L marinobufagenin, or with a combination of ANP and marinobufagenin. Collagen‐1, Fli1, and PKG1 levels were assessed by Western blotting analyses. Vascular PKG1 and Fli1 levels in the old rats were reduced compared with their young counterparts. ANP prevented inhibition of vascular NKA by marinobufagenin in young VSMC but not in old VSMC. In VSMC from the young rats, marinobufagenin induced downregulation of Fli1 and an increase in collagen‐1 level, whereas ANP blocked this effect. Silencing of the PKG1 gene in young VSMC resulted in a reduction in levels of PKG1 and Fli1; marinobufagenin additionally reduced Fli1 and increased collagen‐1 level, and ANP failed to oppose these marinobufagenin effects, similar to VSMC from the old rats with the age‐associated reduction in PKG1. Conclusions Age‐associated reduction in vascular PKG1 and the resultant decline in cGMP signaling lead to the loss of the ability of ANP to oppose marinobufagenin‐induced inhibition of NKA and fibrosis development. Silencing of the PKG1 gene mimicked these effects of aging.