In a controlled study of the influence of hemodialysis on hemodynamics, complement system, leucocyte- and thrombocyte concentration was studied in two groups of five non-uremic dogs each. During the initial 30 min of dialysis with the commonly used cuprophane membrane, a transient but significant decrease of the mean aortic pressure and cardiac output occurred while simultaneously heart rate and systemic vascular resistance decreased. Thrombocyte concentration decreased significantly during the initial 60 min of dialysis due to sequestration in the dialyzer. Transient leucopenia due to pulmonary leucostasis occurred without decrease of the arterial oxygen tension. The simultaneous occurrence and transient character of the hemodynamic changes, leucopenia and thrombocytopenia suggest a relationship between these phenomena. These results indicate that the biocompatibility of cuprophane dialysis membranes is of importance not only with regard to changes in the leucocyte- and thrombocyte-count but also with regard to hemodynamic changes.