The time dependency of rCBF response to photic stimulation in the primary visual cortices was studied using H215O bolus-injection and PET. The subjects consisted of 5 normal volunteers aged from 25 to 40 years old. Their regional CBF was measured at 2 min and 7 min after the start of photic stimulation (frequency, 10 Hz). The percentage increases of rCBF were 19.6 +/- 7.9% (mean +/- SD, range: 8.9-27.6%) in the primary visual cortex on the left side and 23.2 +/- 13.6% (2.5-35.8%) on the right side at 2 min, respectively, while, in comparison, they were 25.3 +/- 20.0% (1.8-49.7%) on the left side and 25.3 +/- 16.4% (3.5-44.5%) on the right side at 7 min, respectively. No significant difference was found in the rCBF responses between at 2 min and 7 min. Therefore, the taming effect does not need to be considered when carrying out a photic stimulation study.