Cardiac fibrosis is a key component of diabetes and involves the proliferation and differentiation of matrix-producing fibroblasts. We determined the influence of high glucose (HG) conditions on cardiac fibroblasts (CF) functions and the effects of recombinant human (rh) relaxin (RLX) in these responses. We cultured neonatal rat CF in either normal glucose (NG) or HG media. The mRNA of procollagen types I and III, and RLX-1 were assessed by real time PCR and procollagen type I C-terminal peptide (PICP) and procollagen type III amino terminal peptide (PIIINP), matrix metalloproteinases 2 (MMP2), MMP9 were assessed by enzyme linked immunosorbent assay. The results are as follows: a) CF proliferation was significantly increased by HG; rhRLX significantly inhibited HG fibroblast proliferation, while it had no marked effect on CF proliferation in NG. b) CF treated with HG significantly increased the production of PICP and PIIINP. rhRLX had no marked effect on production of PICP and PIIINP in NG. rhRLX blocked the HG-induced increases in collagen synthesis. c) The production of MMP2 and MMP9 is significantly increased by HG. rhRLX decreased overproduction of MMP2 and MMP9 in the presence of HG. d) The RLX-1 mRNA expression of HG group was higher than in the NG group. We concluded that rhRLX could inhibit both the proliferation of CF and the synthesis of collagen under the HG condition. HG concentration could stimulate the expression of endogenous RLX. (J. Endocrinol. Invest. 32: 242-247, 2009) (C) 2009, Editrice Kurtis