While recombinant tissue plasminogen activator (rt-PA) is successfully used in human ischemic stroke, it may also cause hemorrhagic complications. Animal experiments have shown that hemorrhages are related to microvascular basal lamina damage. We investigated the effects of different doses of rt-PA on the brain microvasculature. Experimental cerebral ischemia in rats was induced for 3 h and followed by 24 h reperfusion (suture model). Each group of rats (n = 6) received either treatment (0.9, 9, or 18 mg rt-PA/kg body weight) or saline (control group) at the end of ischemia. The loss of microvascular basal lamina antigen collagen type IV was measured by Western blot of the ischemic and non-ischemic basal ganglia and cortex. Compared with the contralateral non-ischemic area, collagen type IV was significantly reduced in the ischemic area: (basal ganglia/cortex) 43% +/- 9% / 64% +/- 4 %. Low/moderate doses of rt-PA had a protective effect: 0.9 mg 79% +/- 3% / 89% +/- 6%, 9 mg 72% +/- 9%/ 81% +/- 12% (p < 0.05). Higher doses of rt-PA (18 mg) had a similar effect as seen in untreated controls: 57% +/- 11% / 59% +/- 9% (p < 0.05, Anova). MMP-9 and MMP-2, measured by gelatine zymography, steadily increased over higher doses of rt-PA: MMP-9 (basal ganglia/cortex): control 115% +/- 4% / 123% +/- 3% compared with 18 mg rt-PA 146% +/- 5%/ 162% +/- 6% (p < 0.05) and MMP-2: control 109% +/- 4%/ 116% +/- 5% and 18 mg rt-PA 222% +/- 15%/ 252% +/- 2% (p < 0.05). Low to moderate doses of rt-PA protect the microvascular basal lamina, whereas high doses of rt-PA have the opposite effect, probably due to increased coactivation of MMP-2 and MMP-9.