In spontaneously breathing rats, a transient increase in cerebral blood flow (CBF) has been shown to be a predictor of hyperbaric oxygen (HBO)-induced convulsion. In the present study, we evaluated whether artificially ventilated animals also show an increase in CBF prior to the onset of HBO-induced convulsion. Rats were ventilated with 100% oxygen in 5 atmospheres. CBF, blood pressure, and an electroencephalogram were monitored continuously. Convulsion was observed at 41 +/- 12 min after the initiation of HBO treatment. A single abrupt increase in CBF, reaching 223 +/- 39% of the control level, was observed at 29 +/- 13 min after the initiation of HBO exposure and lasted until the onset of convulsion 12 +/- 2 min later. The time of the increase in CBF correlated strongly with the onset of convulsion (r = 0.99, P < 0.001). Further, the logistic regression curve demonstrated a close relationship between the duration of increased CBF and percentage of epileptiform electrical-discharge incidence (r = 0.92, P < 0.006). The durations of increased CBF causing convulsion in 10%, 50%, and 90% of the rats were 8.4 min, 11.7 min, and 15.1 min, respectively. These results indicate that an increase in CBF is a predictor of HBO-induced convulsion in artificially ventilated rats. The increase in CBF may be involved in the pathogenesis of HBO-induced convulsion.