Background/Aims: The mineralocorticoid hormone, aldosterone, has pro-fibrotic properties which can cause kidney damage. The severity of kidney interstitial fibrosis is dependent on the accumulation of fibroblasts, which result largely from local proliferation; however, it is unknown whether aldosterone stimulates kidney fibroblast proliferation. Therefore, we examined the effects of aldosterone on the proliferation of cultured kidney fibroblasts. Methods: Uptake of (3)H-thymidine and cell number quantitation were used to determine the proliferative effects of aldosterone on a rat kidney fibroblast cell line (NRK49F cells) and interstitial fibroblasts extracted from mouse kidneys after unilateral ureter obstruction. The role of different mitogenic signalling pathways in aldosterone-induced proliferation was assessed using specific inhibitors of receptors and kinases. Results: Physiological levels of aldosterone induced a doubling of proliferation of kidney fibroblasts (p <0.0001), which was inhibited by pre-treatment with the mineralocorticoid receptor antagonist, eplerenone. Aldosterone-induced fibroblast proliferation was dependent upon the kinase activity of growth factor receptors [platelet-derived growth factor receptor (PDGFR) and epidermal growth factor receptor]. Notably, PDGF ligands were not involved in aldosterone-induced PDGFR activation, indicating receptor transactivation. Aldosterone-induced fibroblast proliferation also required signalling via PI3K, JNK and ERK pathways, but not via the transforming growth factor-beta(1) receptor. Conclusion: Aldosterone ligation of the mineralocorticoid receptor in kidney fibroblasts results in rapid activation of growth factor receptors and induction of PI3K/MAPK signalling, which stimulates proliferation. This suggests that increased levels of aldosterone during disease may promote the severity of kidney fibrosis by inducing fibroblast proliferation.