The integrity of the mesothelial layer is essential for both defence and solute transport in continuous ambulatory peritoneal dialysis (CAPD). The human peritoneal mesothelial cell (HPMC) culture has been shown to be a very useful tool to study the peritoneal mesothelial stem cell behaviour. We investigated whether hydralazine, an antihypertensive agent frequently used, might affect HPMC growth and collagen synthesis. HPMCs were cultured from specimens of human omentum by enzymatic disaggregation of omentum. HPMC growth was evaluated by modified methyltetrazolium (MTT) assay. Cell viability was confirmed by trypan blue exclusion and lactate dehydrogenase assay. Collagen synthesis was measured by 3H-proline incorporation into pepsin-resistant, salt-precipitated collagen. Intracellular cAMP levels were measured by enzyme immunoassay. The procollagen alpha 1 (I) mRNA expression was evaluated by Northern blot analysis. Hydralazine inhibited serum-stimulated HPMC growth in a dose-dependent manner. The maximal inhibition was 93% at a concentration of 100 micrograms/ml. Hydralazine inhibited collagen synthesis in confluent mesothelial cells (47% inhibition at a concentration of 100 micrograms/ml). The procollagen alpha 1 (I) mRNA expression was also decreased by hydralazine (about 50% decrease at 100 micrograms/ml). These effects may be due to the phosphodiesterase inhibition property of hydralazine to increase intracellular cAMP levels. These data suggest that the use of hydralazine in CAPD patients may affect peritoneal membrane function and integrity.