Indocyanine green (ICG) has excellent safety records and is widely used in medical diagnosis. Recently, a new method has been developed to estimate cerebral blood flow (CBF) using ICG in combination with near-infrared spectroscopy (NIRS). The new technique may be of wide clinical interest, as it is noninvasive and easy to perform at the bedside in stroke patients. Additionally, ICG with the use of specific wavelength lasers is documented to be effective in photodynamic therapy (PDT). Under normal conditions ICG does not cross the intact blood brain barrier (BBB). However, in patients with brain injuries where the BBB may be disturbed, ICG could accumulate in brain parenchyma and in combination with NIR-light exposure, phototoxicity could occur. The aim of the present study was to examine the possible toxicity of ICG in combination with NIRS in a specific setting for CBF measurements. In five rats with mannitol induced BBB breakdown no traces of ICG were found during spectrophotometric analysis of the brain cell suspensions. In ten rats with disrupted BBB there were no significant increases of brain temperature or histological signs of brain damage following 1 h NIR-light exposure after ICG injection. The existing literature concerning the application of ICG in combination with NIR light is reviewed. © 2002 Elsevier Science B.V. All rights reserved.