The effects of hypoxic hypoxia on local cerebral blood flow was studied by using 14C-iodoantipyrine autoradiography. Hypoxia (8%O2 and 6%O2 for 30min) was induced in mice without control of the temperature and blood pressure. Percent decreases in cerebral oxygen tension (P cortex O2) during 8%O2 and 6%O2 hypoxia were 80% and 92%, respectively. Under normal conditions (21%O2), P cortex O2 was 17.4±4.0mmHg. Optical density (OD), main factors responsible for the measurement of local cerebral blood flow were observed by the scanning micro-densitometer. The pyramidal cell layer of the hippocampus, choroid plexus, and venous (sinus) blood did not decrease in ODs during hypoxic hypoxia, while ODs in all other cerebral regions examined significantly decreased as compared with that of the control (P<0.01). In others, amygdala showed a marked decrease (ca. 30%) in ODs during hypoxic hypoxia.