The course of fibrinogen (Fgen), red cell aggregation (RCA), plasma viscocity (PV), platelet aggregation (PA) and hematocrit (Hc) was studied in patients with ultrahigh - dose thrombolytic therapy (1.5 x 10(6) units/hour for 6 hours = 1 cycle) with streptokinase (SK) or urokinase (UK) over a period of 3 cycles. Both ultrahigh - dose SK and UK produced significant changes in the course of Fgen, RCA and PV, whereas PA (spontaneous and ADP-induced) and Hc remained unchanged. After termination of each cycle Fgen progressively increased while RCA and PV further decreased. The extent of alteration in cycle 1 - concerning the baseline values - was more pronounced with SK than with UK, but the overall effect of SK decreased through the consecutive cycles because of more rapid increase during the SK-free period. In UK-therapy hemorheological alterations were initially moderate but increased from cycle to cycle.