The authors tested in vitro the effect of glucose-based and amino acid-based dialysate effluent on the function of human peritoneal mesothelial cells. After 9 days of exposure to the tested effluents with medium (1:1 v/v) or to a medium supplemented with 10% fetal calf serum (FCS) (control), several functional properties of the cells were studied. The synthesis of DNA measured by incorporation of 3H-methyl-thymidine was higher in mesothelial-cell monolayers exposed to the dialysates than in the controls. Synthesis of hyaluronic acid was similar in all three groups, but after stimulation with Il-1 the cells exposed to the dialysates produced more hyaluronic acid. Synthesis of tissue plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) was higher in the control cells. However, after stimulation with IL-1, the cells exposed to the dialysate showed greater synthesis of PAI-1 than of t-PA. Also, procoagulant activity of the control cells was higher than that of the cells exposed to the dialysates. We have concluded that the functional properties of the mesothelial cells may be altered in vitro during prolonged exposure to the dialysate, something that may also occur in vivo.