Heparin given over a long term by a subcutaneous route consistently lowers blood pressure in the hypertensive rat models. The decrease in blood pressure is accompanied by a parallel decrease in hematocrit suggesting a causal relationship between hematocrit and blood pressure. The aim of this study was to define the relationships between acute and chronic hematocrit changes and blood pressure in the normotensive and hypertensive states. Normotensive Wistar (NWR) and spontaneously hypertensive (SHR) rats were used. Hematocrit was decreased acutely by blood-letting, and chronically by treatment with either heparin (H) or phenylhydrazine (P) for 4 weeks. Acute and chronic hematocrit increase was accomplished by packed cells transfusion. Systolic blood pressure was measured weekly; and at the end of the experimental period, plasma volume, cardiac output, and mean arterial pressure were obtained. Acute hematocrit decrease or increase (hematocrit ranging from 25 to 65%) did not affect blood pressure in either strain of rats; whereas chronic hematocrit changes (hematocrit ranging from 35 to 61%) significantly affected blood pressure only in SHR. Thus, chronic hematocrit decrease induced by H or P resulted in a significant fall in blood pressure compared to control (201 +/- 3 v 175 +/- 4, 167 +/- 4 mm Hg, respectively; P < .05). Conversely, a chronic hematocrit increase resulted in a significant rise in blood pressure (201 +/- 3 v 219 +/- 4 mm Hg; P < .05). Similar hematocrit changes produced in NWR, as in SHR, did not affect blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)