A single 18 mg/kg dose of oral phenytoin capsules or suspension (mean dose, 1.3 g) was given to 44 patients with recent seizures and no detectable serum phenytoin level. Mean serum phenytoin levels after loading for patients receiving capsules were 6.8 micrograms/mL at two hours, 9.7 micrograms/mL at three to five hours, 12.3 micrograms/mL at six to ten hours, and 15.1 micrograms/mL at 16 to 24 hours. Mean levels for patients receiving suspension were slightly, but not significantly, lower than for patients receiving capsules. No seizures occurred during an eight-hour observation period after loading. Drug toxicity was minimal. Single-dose, 18 mg/kg oral phenytoin loading provides rapid therapeutic levels and is well tolerated.