Early seizures represent a major complication in the post operative course of patients operated on for supratentorial tumors or AVMs. The real effectiveness of the AEDs prophylaxis to reduce the occurrence of post operative seizures is controversial. We proposed a prophylactic treatment with endovenous PHT consisting of two infusions of PHT (mean dosage of 18 mg/kg; mean time of 1 hr) perioperatively and during the first postoperative day. The interruption of the previous oral anticonvulsant treatment is not required. The endovenous route should permit a rapid reach of the therapeutical range. Sixty-six patients were treated. Fifty-one patients received two infusions and 15 patients only one infusion. The serum concentration of PHT performed at 24 hrs of operation was in most of patients (more than 80%) in the lower part of the therapeutical range while at 24 hrs of the second infusion was in the higher part or over the range. The overall prevalence of seizures was 10.6%. In the first group the incidence was 7.8%, in the second one was 20%. All the seizures appeared within 48 hrs of the operation. All the patients in the first group had single seizures, 2 patients of the second one experienced two seizures. No status epilepticus was observed. Alteration of consciousness and mild hypotension were the most common side effects. They never required major measurements and were mild, transient and completely reversible. We are starting with a randomized study based on a larger sample of patients which will allow a more reliable statistical analysis.