Previous studies have demonstrated a subset of normotensive rats (Wistar-Kyoto, WKY) that has arteriovenous shunts and biventricular hypertrophy. In this study, using the combined radioactive reference sample and microsphere techniques, spontaneously hypertensive (SHR) and WKY rats were found to have arteriovenous fistulas, and the magnitude of this shunting was quantified. In both groups with shunts (WKY and SHR, respectively) mean arterial pressure was lower (117 +/- 3 vs. 125 +/- 2 mmHg, NS; 164 +/- 4 vs. 184 +/- 7 mmHg, P less than 0.001) and cardiac index was higher (518 +/- 52 vs. 259 +/- 10 ml.min-1.kg-1, P less than 0.001: 416 +/- 18 vs. 239 +/- 8 ml.min-1.kg-1, P less than 0.001) than in rats without shunts. Both groups demonstrated sizable shunts, with microsphere trapping in 55 +/- 6 and 47 +/- 5% in WKY and SHR, respectively, and reduced blood flow to every organ. These data demonstrate the need for caution in evaluating hemodynamic and myocardial functional data as well as endocrinologic or metabolic functions in WKY and SHR unless shunting is excluded. Moreover, since these subsets seem to have had arteriovenous shunts from birth, it may be inferred that the development of the arterial hypertension in SHR did not result from tissue hyperperfusion.