Aldosterone is thought to modulate renal fibrosis, in part, through increasing plasminogen activator inhibitor type 1 (PAI-1), a major inhibitor of ECM degradation. The present study investigated aldosterone effects on PAI-1 and transforming growth factor-beta (TGF)-beta1, asked whether PAI-1 effects were TGF-beta-mediated and whether aldosterone and TGF-beta 1 acted synergistically to increase PAI-1 and decrease ECM degradation. Rat mesangial cells (MCs) and fibroblast cells (NRK-49F) were used. 3H-labeled ECM was produced by MCs. The effect of aldosterone and TGF-beta on ECM degradation by newly-plated MCs or NRK-49F was measured by the release of 3H into medium. Aldosterone markedly increased PAI-1 mRNA and protein in both cell types, increases completely blocked by spironolactone and partially blocked by TGF-beta neutralizing antibody. Adding both aldosterone and TGF-beta1 produced PAI-1 mRNA and protein increases higher than the sum of increases seen with either compound alone. Aldosterone or TGF-beta1 alone inhibited matrix degradation by 39% and 49% in MCs and 21% and 23% in NRK-49F, respectively. When both compounds were added, matrix degradation was further decreased by 93% in MCs and 61% in NRK-49F. The results indicate that aldosterone-induced PAI-1 increases are partially mediated by TGF-beta1 and lead to decreased ECM degradation. While aldosterone alone induced TGF-beta1 weakly, aldosterone and TGF-beta1 added together produced significant synergistic effects on PAI-1 production and subsequent ECM accumulation. Thus, the elevated aldosterone induced by RAAS activation may amplify RAAS profibrotic actions.