Consideration of the interactions of drugs and dialysis must include an understanding of the mechanisms of transport during dialysis, i.e., diffusion, ultrafiltration and membrane-protein binding effects. Clearance is a function of molecular size, blood and dialysate flow, membrane area and permeability, and dialyzer support geometry. Protein binding and hematocrit decrease the in vivo clearances in comparison to those measured in vitro with aqueous solutions. The effect on the serum half-life is also determined by the distribution space and clearance by other routes. Other factors such as metabolic alterations of dialysis can affect pharmacologic activity, and the clinical response is the end product of many determinants. Numerous drugs are effectively removed by hemodialysis or at a slower rate by peritoneal dialysis, which occasionally allows considerable influx. The influence of intestinal contents on elimination rates by peritoneal dialysis is unknown. Peritoneal dialysis can be influenced considerably by vasoactive drugs.