Behavioral state-induced changes in fetal cerebral blood flow were continuously monitored with a simple thermal dilution method. Thermojunctions were heated 1.5 degrees C above reference thermojunctions implanted contralaterally in various cerebral cortical and subcortical structures of four near-term fetal sheep. Temperature difference in rapid-eye-movement sleep was lower than in non-rapid-eye-movement sleep (p = 0.014), reflecting convective heat loss from increased blood flow. Temperature difference also varied significantly with the locus of placement in the brain (p = 0.003), reflecting, in part, regional differences in cerebral blood flow. The thermocouple method gives qualitative, continuous information on local cerebral blood flow that could be useful in monitoring the vascular response to changing functional activity during prenatal brain development.