Phenytoin was administered intravenously in large doses (mean = 16.6 mg per kilogram) for prevention and treatment of seizures on 159 occasions to 139 patients aged 17 to 94 years (mean = 52 years) and weighing 37 to 113 kg (mean = 65 kg). Hypotension was more frequent among older patients. No deaths were attributable to phenytoin. Volumes of distribution were relatively constant (mean = 0.78 +/- 0.11 liters per kilogram), but half-lives varied considerably and were prolonged (mean = 51 +/- 32 hours) because of the large doses administered. A dose of 18 mg per kilogram was effective in maintaining phenytoin serum levels above 10 microgram per milliliter for 24 hours.