Indocyanine green (ICG) is a near-infrared dye that has the potential to beused as a tracer for the minimally invasive measurement of cerebral bloodflow (CBF). In order to examine the technique, the arterial and cerebralconcentrations of ICG were measured in newborn piglets during the boluspassage of ICG at normocapnia and two levels of mild hypercapnia. Theresults were analysed by applying the Fick principle in both integral anddifferential forms using a linear regression technique to improve theprecision of calculated values of CBF.It was found that the integral method, which has been used previously, isparticularly sensitive to errors in the time registration between thearterial and tissue signals whereas the differential method is less so. Inaddition, the differential method allows the venous outflow to becalculated which gives further information on the state of the capillarybed. CBF was 39.7±4.6 ml 100 g-1 min-1 at anarterial carbon dioxide tension (PaCO2) of 33.0±2.2 mmHg andincreased to 53.7±9.1 and 75.4±15.2 ml 100 g-1 min-1 at a PaCO2 of 42.1±2.6 and 54.2±3.1 mmHg respectively (mean ± SD, n = 7). There was nosignificant change in cerebral metabolic rate for oxygen, validating thevalue of blood flow to an arbitrary scaling factor. When the inspiredCO2 fraction was returned to zero, calculated CBF returned to baselinewith a variation of 7% of the mean, indicating that this technique ishighly precise.