Possible relationships between the dialysate-to-plasma creatinine equilibration ratio (D/Pcreatinine 4 hour), duration of peritoneal dialysis treatment, number of peritonitis episodes, and mass appearance rates of three connective tissue markers [carboxyterminal propeptide of type I procollagen (PICP), aminoterminal propeptide of type III procollagen (PIIINP), and carboxyterminal telopeptide of type I collagen (ICTP)] were studied in 19 nondiabetic peritoneal dialysis patients. The absence of correlation between the mass appearance rates of the markers and the duration of dialysis treatment as well as the number of peritonitis episodes supports the concept that peritoneal dialysis does not cause persistent changes in the deposition and degradation rates of collagen. A correlation between the D/Pcreatinine 4 hr and the PICP mass appearance rates was found. Since it is unlikely that transperitoneal transport alone is responsible for the appearance of PICP in dialysate, this might reflect an association between a large peritoneal surface area and the amount of submesothelial connective tissue.