The authors studied the in vitro effect of overnight "dwelled" dialysate obtained from 5 continuous ambulatory peritoneal dialysis (CAPD) patients on the growth of, and protein synthesis by, peritoneal mesothelial cells and fibroblasts. In each patient, an overnight exchange was performed with Dianeal 2.5% on day 1 and with a solution containing amino acids 1.1% on day 2. Both types of dialysate supported the proliferation of fibroblasts and mesothelial cells, but the amino acid dialysate had a stronger effect on the growth of mesothelial cells than the Dianeal dialysate did. In the presence of either dialysate, synthesis of noncollagen proteins by mesothelial cells and fibroblasts was low. By contrast, the synthesis of collagen was enhanced, and the Dianeal dialysate had a much stronger effect than the amino acid dialysate did. Present results show that, during its intraperitoneal dwell, dialysis fluid within the peritoneal cavity stimulates the generation of bioactive substances which affect the function of peritoneal cells. This reaction is influenced by the composition of the dialysis solution.