During 7 months of calcitriol treatment in eight CAPD patients, significant increases in serum creatinine and daily excretion of this substance in dialysis fluid were observed, but there was a significant decrease in creatinine in the urinary daily output. When therapy was stopped, creatinine levels fell to baseline in serum and peritoneal fluid, and urine values continued to decrease as previously observed. Dynamometric performance showed a significant increase during treatment, which remained after drug withdrawal. These findings suggest that the transient increase of serum creatinine during calcitriol treatment may be a result of an increased production and release of this metabolite from muscle.