We studied the hypotensive effect of a rapid intravenous infusion of high-dose streptokinase in 98 patients with an acute myocardial infarction. The systolic blood pressure fell from 132 +/- 20 (range 90 to 174) to 97 +/- 21 mm Hg (range 58 to 152) at 15 +/- 8 min (range 4 to 40) after the commencement of the streptokinase infusion (p less than .001). A fall in diastolic blood pressure from 80 +/- 16 (range 51 to 105) to 61 +/- 15 mm Hg (range 32 to 92) accompanied the fall in systolic pressure (p less than .001). The fall in blood pressure was associated with an increase in heart rate (73 +/- 14 to 78 +/- 17 beats/min, p less than .001), preceded the appearance of clinical signs of reperfusion by 37 +/- 38 min and was similar in magnitude and timing in patients with anterior and inferior infarction. There were direct relationships between the rate of infusion of streptokinase and both the magnitude (r = .49, p less than .001) and the rate of fall of systolic blood pressure (r = .67, p less than .001) as well as both the magnitude and rate of fall of diastolic blood pressure. In most patients, the fall in blood pressure was transient (9 +/- 6 min, range = 2 to 30) and easily managed by slowing or stopping the infusion, placing the patient in the Trendelenburg position, or by administering an infusion of low-dose norepinephrine or dopamine. However, in four patients with severe left ventricular dysfunction, severe hypotension persisted for more than 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)