The present study investigates the effects of polycythemia on renal hemodynamics and function in 15 anesthetized newborn dogs, 2-10 days of age. Microspheres were used to study renal blood flow. Experimental animals received an exchange transfusion with adult dog packed red blood cells resulting in an increase in hematocrit from 38% +/- 2.1% to 69% +/- 2.1%. Control animals received an exchange transfusion with adult dog whole blood so that there was no change in hematocrit. Polycythemia resulted in a marked increase in blood viscosity, a 40% fall in cardiac output from 251.8 +/- 14 ml/kg/min to 150.7 +/- 9 ml/kg/min and a 98% increase in total vascular resistance from 0.32 +/- 0.02 mmHg/ml/kg/min to 0.63 +/- 0.07 mmHg/ml/kg/min. Nevertheless, renal blood flow was not significantly altered indicating renal vasodilation (1.4 +/- 0.06 ml/g/min initial versus 1.2 +/- 0.09 ml/g/min final). Although renal blood flow was well preserved, renal plasma flow decreased by 63% as the hematocrit increased from 0.86 +/- 0.03 ml/g/min to 0.38 +/- 0.04 ml/g/min, resulting in a 53% fall in glomerular filtration rate from 0.21 +/- 0.02 ml/min/g kidney weight to 0.09 +/- 0.02 ml/min/g kidney weight. There was also a large drop in urine output and Na and K excretion following polycythemia. This was due primarily to the decreased filtered load, because fractional Na reabsorption remained constant. Thus, in spite of well-preserved renal blood flow, polycythemia markedly affected renal function, resulting in water and salt retention.