Plasma viscosity (PV) was examined in healthy (n = 122) and sick (n = 58) dogs. The mean value of PV in healthy adult dogs is 1.22 mPas. PV is remarkably increased in case of illness. In dogs with chronic heart failure the mean value was 1.46 mPas. Patients suffering from uraemia or pyometra revealed mean values of 1.90 mPas and 1.96 mPas respectively. An elevation of the PV is correlated with an increase of the "acute-phase-proteins". The examination of PV is useful for a better assessment of microcirculation. The PV is a parameter, which is important in diagnostics as well as in follow-up during special therapeutical procedures.