Internal carotid artery (ICA) blood flow, mean arterial blood pressure (MABP), intracranial epidural pressure (EDP), and arterial blood gases were measured during spontaneous, generalized epileptic seizures in man. Concomitant with the clinical seizure there was a nearly threefold increase in ICA blood flow mainly caused by a rapid fall in cerebrovascular resistance (CVR). As changes in arterial blood gases after 30–45 sec of apnea and convulsions were moderate, the initial fall in CVR was attributed to local cerebral changes of either metabolic or neurogenic nature. Changes in MABP showed two different patterns during seizures, pattern 1 with a transient increase in pressure, and pattern 2 with a marked fall followed by an overshoot in pressure. The fall in MABP was probably caused by a Valsalva effect or a central nervous system vasodepressor response.