Background. After long-term treatment with continuous ambulatory peritoneal dialysis (CAPD), some patients may develop peritoneal fibrosis. Peritoneal mesothelial cells (PMCs) participate in the inflammatory reactions in the peritoneal cavity, and transforming growth factor-beta 1 (TGF-beta 1) and interleukin-1 beta (IL-1 beta) are involved in peritoneal fibrosis. Diltiazem is used frequently in patients with CAPD to treat hypertension. The objectives of this study were to examine the effects of diltiazem on collagen- and IL-1 beta-induced TGF-beta 1 production on human PMCs and the signalling pathway of diltiazem in this induction. Methods. Human PMCs were cultured from the enzymatic disaggregation of human omentum. Collagen synthesis was measured by [H-3]proline incorporation into pepsin-resistant, salt-precipitated collagen. The expression of collagen I and III, and TGF-beta 1 mRNA was evaluated by northern blotting. The production of TGF-beta 1 by human PMCs was measured by immunoassay. The changes of intracellular calcium level after adding Fura-2-AM were measured by fluorescence spectrophotometry. Western blotting was used to assess mitogen-activated protein kinase (MAPK) signalling proteins. Results. We found that diltiazem (< 0.2 mM) inhibited collagen I and III mRNA expression and collagen syntheses on a dose-dependent basis. Diltiazem (0.2 mM) suppressed IL-1 beta- (5 ng/ml) induced TGF-beta 1 production on human PMCs at both the protein and mRNA levels. Diltiazem (0.2 mM) also inhibited IL-1 beta- (5 ng/ml) induced collagen I and III mRNA expression. Intracellular calcium levels did not change after the treatment with diltiazem, IL-1 beta or both. The IL-1 beta-treated human PMCs increased phospho-JNK (stress-activated c-Jun N-terminal kinase) and phospho-p38 MAPK expression, while diltiazem could suppress this phenomenon. Conclusions. Diltiazem suppressed collagen synthesis of human PMCs and inhibited IL-1 beta-induced TGF-beta 1 production on human PMCs. This signalling transduction may be through p38 MAPK and JNK pathways instead of intracellular calcium. These results suggest diltiazem to be a potential therapeutic regimen in preventing peritoneal fibrosis and support further in vivo studies.