Toxic cyanobacteria in public water reservoirs may cause severe health issues for livestock and human beings. Geitlerinema amphibium, which is frequently found in São Paulo City's drinking water supplies, showed toxicity in the standard mouse bioassay, while displaying signs of intoxication and post-mortem findings different from those showed by animals intoxicated by known cyanotoxins. We report here the alterations caused by G. amphibium methanolic extract on mouse microcirculatory network, as seen by in vivo intravital microscopy, besides observations on leukocyte migration, cytokine quantitation, and results of toxicological essays. Our data showed that G. amphibium methanolic extract displayed time- and dose-dependent pro-inflammatory activity, and that at lower doses [125 and 250 mg/kg body weight (b.w.)] increased the leukocyte rolling, caused partial venular stasis, as well as induced an increase in leukocyte counts in the peripheral blood and peritoneal washings. At higher doses (500 and 1000 mg/kg b.w.), the extract caused ischemic injury leading to animal death. As confirmed by mass spectrometric studies and polymyxin B test, the G. amphibium methanolic extract did not contain lipopolysaccharides. © 2011 Elsevier Ltd.